SUMMARY

        This summary highlights selected information from this document. It does not contain all of the information that may be important to you. You should carefully read this entire document and the other documents referred to in this document before you decide how to vote. Together these documents will give you a more complete description of the proposed transaction. Page references are included in this summary to direct you to more thorough descriptions of the topics provided elsewhere in these materials.

The Special Meeting of Stockholders

New Windsor special meeting of stockholders to be held May 16, 2017 (see page 42).

        New Windsor will hold a special meeting of stockholders on May 16, 2017, at 10:00 a.m., local time, at New Windsor Fire & Hose Company, Windsor Station, 101 High Street, New Windsor, Maryland 21776.

New Windsor record date set at March 17, 2017; one vote per share of New Windsor common stock (see page 42).

        If you owned shares of New Windsor common stock at the close of business on March 17, 2017, you are entitled to notice of, and to vote at, the special meeting of stockholders. You will have one vote at the special meeting for each share of New Windsor common stock you owned on March 17, 2017. On March 17, 2017, there were 1,008,422 shares of New Windsor common stock outstanding.

The Companies

ACNB Corporation
16 Lincoln Square
Gettysburg, Pennsylvania 17325
(717) 334-3161

        ACNB Corporation, with assets of $1.2 billion as of September 30, 2016, is the financial holding company for ACNB Bank. ACNB stock is quoted on The NASDAQ Capital Market under the symbol "ACNB." For more information, visit the ACNB Bank website at www.acnb.com. The information on ACNB's website is not incorporated into this proxy statement/prospectus.

New Windsor Bancorp, Inc.
222 E. Baltimore Street
Taneytown, MD 21787
(410) 549-1414

        New Windsor Bancorp, Inc., with assets of $311.1 million as of September 30, 2016, is the holding company for New Windsor State Bank. New Windsor stock is quoted on the OTC Pink market place under the symbol "NWID." For more information, visit the New Windsor website at www.newwindsorbank.com. The information on New Windsor's website is not incorporated into this proxy statement/prospectus.

We propose that New Windsor merge into a wholly owned subsidiary of ACNB (see page 44).

        Under the terms of the reorganization agreement, ACNB will acquire New Windsor by merging New Windsor with and into a wholly owned subsidiary of ACNB. New Windsor will cease to exist as a separate entity. Following the merger, New Windsor State Bank will merge with and into ACNB Bank. The subsidiary surviving the New Windsor merger will also dissolve and liquidate into ACNB, so that all of the assets and liabilities of New Windsor will become assets and liabilities of ACNB. A copy of

the reorganization agreement is attached to this document as Annex A and a copy of the bank plan of merger is attached to the reorganization agreement as Exhibit G.

New Windsor stockholders will receive 1.10 shares of ACNB common stock or $30.00 for each share of New Windsor common stock (see page 70).

        If the merger is completed, each share of New Windsor common stock will be converted into the right to receive either 1.10 shares of ACNB common stock or $30.00 in cash. The stock exchange ratio will also be appropriately adjusted if there is a stock dividend, stock split, reverse stock split, common stock reclassification, or other similar event regarding ACNB common stock before completion of the merger. New Windsor stockholders also will receive cash instead of any fractional shares they would have otherwise received in the merger.

        Each holder of New Windsor common stock will be able to elect to receive ACNB common stock, cash, or a combination of cash and ACNB common stock for his or her shares of New Windsor common stock. Regardless of your choice, however, elections will be limited by the requirement that 85% of the shares of New Windsor common stock be exchanged for ACNB common stock and 15% be exchanged for cash. Therefore, the allocation of ACNB common stock and cash that you receive will depend on the elections of other New Windsor stockholders.

        Immediately following the merger, former New Windsor stockholders are expected to own approximately 13.5% of total outstanding ACNB common stock.

New Windsor's board of directors has received an opinion from its financial advisor regarding the merger consideration to be received by the holders of New Windsor common stock (see page 55).

        In connection with the merger, the board of directors of New Windsor received a written opinion from New Windsor's financial advisor, Sandler O'Neill & Partners, L.P. ("Sandler O'Neill"), as to the fairness, from a financial point of view and as of the date of such opinion, of the merger consideration to be received by the holders of New Windsor common stock. The full text of the opinion of Sandler O'Neill, dated November 21, 2016, is included in this document as Annex B. New Windsor encourages you to read this opinion carefully in its entirety for a description of the procedures followed, assumptions made, matters considered, and limitations of the review undertaken by Sandler O'Neill. The opinion of Sandler O'Neill is directed to New Windsor's board of directors and does not constitute a recommendation to any stockholder as to how to vote with respect to the merger, what form of consideration to elect in the merger, or any other matter relating to the proposed merger.

New Windsor stockholder vote required (see page 42).

        Approval and Adoption of the Reorganization Agreement.    The affirmative vote, in person or by proxy, of at least two-thirds of the outstanding shares of New Windsor common stock on the record date is required to approve and adopt the reorganization agreement.

        Discretionary Authority to Adjourn or Postpone the Special Meeting.    In accordance with New Windsor's bylaws, the affirmative vote of a majority of New Windsor shares represented, in person or by proxy, at the special meeting is required to approve the proposal to adjourn or postpone the special meeting, if necessary, to solicit additional proxies in the event that there are not sufficient votes at the time of the special meeting to approve the proposal to approve and adopt the reorganization agreement.

        Each holder of shares of New Windsor common stock outstanding on the record date will be entitled to one vote for each share held of record. The vote required for approval and adoption of the reorganization agreement is a percentage of all outstanding shares of New Windsor common stock. Therefore, abstentions will have the same effect as a vote against the reorganization agreement and the

merger. Brokers who hold New Windsor common stock as nominees on your behalf will not have authority to vote your shares with respect to the reorganization agreement or the merger unless you provide voting instructions in accordance with the directions provided by your broker. Failure to provide your broker with voting instructions will have the same effect as a vote against the reorganization agreement.

New Windsor directors and executive officers have agreed to vote in favor of the merger (see page 93).

        On March 17, 2017, the directors and executive officers of New Windsor had sole or shared voting power over 107,491 shares, or 10.7%, of the outstanding shares of New Windsor common stock. Certain of these directors and executive officers of New Windsor having voting power over 9.2% of the outstanding shares of New Windsor common stock have agreed with ACNB to vote these shares of New Windsor common stock in favor of approving and adopting the reorganization agreement.

New Windsor directors and executive officers may have interests in the merger that differ from your interests (see page 89).

        In considering the information contained in this proxy statement/prospectus, you should be aware that New Windsor's directors and executive officers have financial interests in the merger that are different from, or in addition to, the interests of New Windsor stockholders generally. These interests include, among other things:

•

Tom N. Rasmussen, President and Chief Executive Officer of New Windsor, will receive a new employment agreement providing for certain benefits, an increase in base salary, and a formulaic bonus structure;



•

Lisa Monthley, Chief Deposit Officer of New Windsor State Bank, will receive a two year change in control agreement providing for certain benefits upon a change in control of ACNB and a release agreement providing for certain benefits upon the termination of Ms. Monthley within two years of the merger without cause;



•

New Windsor may award retention bonuses to certain employees;



•

the accelerated vesting of company contributions to the New Windsor Bancorp, Inc. 2005 Deferred Compensation Plan;



•

certain employees will receive ongoing life insurance with a $15,000 death benefit;



•

the right of certain executive officers to receive cash severance under their employment or change in control agreements with New Windsor if their employment is involuntarily terminated without cause or the executive terminates for good reason;



•

the right to continued indemnification and liability insurance coverage for New Windsor's current directors by ACNB after the merger for acts or omissions occurring before the merger; and



•

the appointment Todd L. Herring and D. Arthur Seibel, Jr. to ACNB's board of directors following completion of the merger, and any related compensation for such services.

        None of ACNB's executives and employees will receive change in control related payments or benefits since the merger does not constitute a "change in control" for purposes of ACNB's employment agreements, change in control agreements, equity incentive plan, and the 2009 ACNB Corporation Restricted Stock Plan.

        New Windsor's board of directors was aware of these interests and considered them in approving and recommending the reorganization agreement. These and certain other additional interests of New Windsor's directors and executive officers are described in detail in "Proposal 1—The Merger—

Interests of Directors and Executive Officers in the Merger," found elsewhere in this document. These circumstances may cause some of New Windsor's directors and executive officers to view the proposed merger differently than you view it.

ACNB and New Windsor must obtain regulatory approval and satisfy other conditions before the merger is complete (see page 87).

        ACNB's and New Windsor's obligations to complete the merger and the bank merger are subject to various conditions that are usual and customary for this kind of transaction. These conditions include obtaining approval from the Federal Reserve Board, the FDIC, the Pennsylvania Department of Banking and Securities, and the Maryland Commissioner. As of the date of this document, appropriate applications for approval have been filed. In addition to the required regulatory approvals, the merger will only be completed if certain conditions are met. See "Proposal 1—The Merger—Terms of the Merger—Conditions to Merger."

Amendment or termination of the reorganization agreement is possible (see page 84).

        ACNB and New Windsor can agree to amend the reorganization agreement in any way, except that, after approval by New Windsor stockholders at its special meeting, ACNB and New Windsor cannot change the amount of merger consideration New Windsor stockholders will receive in the transaction from what is provided in the reorganization agreement.

        ACNB and New Windsor may agree to terminate the reorganization agreement and not complete the merger at any time before the merger is completed. Each company also may unilaterally terminate the reorganization agreement in certain circumstances. This includes, among others, the failure to complete the merger by November 30, 2017, unless the terminating company's breach is the reason the merger has not been completed. See "Proposal 1—The Merger—Terms of the Merger—Termination."

Rights of New Windsor stockholders differ from those of ACNB shareholders (see page 137).

        When the merger is complete, New Windsor stockholders who receive shares of ACNB common stock will become ACNB shareholders by operation of law. The rights of New Windsor stockholders differ from the rights of ACNB shareholders in certain important ways. Many of these differences have to do with provisions in New Windsor's articles of incorporation and bylaws that differ from those of ACNB. See "Comparison of Shareholders' Rights."

Material U.S. federal income tax consequences of the merger (see page 94).

        The merger is intended to qualify as a tax free "reorganization" within the meaning of Section 368(a) of the Code. Accordingly, New Windsor stockholders generally will not recognize any gain or loss for U.S. federal income tax purposes on the exchange of shares of New Windsor common stock for shares of ACNB common stock in the merger, except with respect to any cash received in lieu of fractional shares of ACNB common stock. New Windsor stockholders that receive only cash in exchange for their New Windsor common stock will recognize gain or loss on the transaction, and New Windsor stockholders that receive a combination of cash and ACNB common stock in exchange for their shares of New Windsor common stock will typically recognize gain (but not loss) on the transaction. New Windsor stockholders will recognize gain or loss in connection with cash received in lieu of fractional shares of ACNB. This tax treatment may not apply to all New Windsor stockholders.

        It is a condition to the closing of the merger that ACNB receive the opinion of its special counsel, Bybel Rutledge LLP, and that New Windsor receive the opinion of its special counsel, Miles & Stockbridge P.C., substantially to the effect that, on the basis of facts, representations, and assumptions set forth or referred to in that opinion (including factual representations contained in certificates of officers of ACNB and New Windsor), the merger will be treated as a reorganization within the

meaning of Section 368(a) of the Code. The condition is waivable, and in such case, ACNB and New Windsor will undertake to recirculate and resolicit if the condition is waived by either party and the change in the tax consequences is material.

        New Windsor and ACNB urge you to consult your tax advisor for a full understanding of the specific tax consequences of the merger to you. Tax matters are very complicated and, in many cases, tax consequences of the merger will depend on your particular facts and circumstances. See "Proposal 1—The Merger—Material U.S. Federal Income Tax Consequences," beginning at page 94.

New Windsor stockholders are entitled to appraisal rights (see page 98).

        New Windsor stockholders may object to the merger and, upon complying with the requirements of Maryland law, receive cash in the amount of the fair value of their shares instead of shares of ACNB common stock and/or the cash consideration specified in the reorganization agreement. A copy of the section of the MGCL pertaining to objecting stockholders' rights of appraisal is attached as Annex C to this proxy statement/prospectus. You should read the statute carefully and consult with your legal counsel if you intend to exercise these rights. The failure to comply with the statute exactly will result in the loss of your rights as an objecting stockholder.

New Windsor offices will maintain the New Windsor name after the merger (see page 83).

        For at least two years after the effective time of the merger, the former New Windsor State Bank branches will operate under the name "NWSB Bank, a division of ACNB Bank" unless the board of directors of ACNB shall determine otherwise upon approval of at least 80% of the ACNB board in connection with an acquisition of a Maryland bank or the acquisition of ACNB.

Market Price and Dividend Information

ACNB

        A substantial source of ACNB's income from which it can pay dividends is the receipt of dividends from ACNB Bank. The availability of dividends from ACNB Bank is limited by various statutes and regulations. It also is possible, depending on the financial condition of ACNB Bank, and other factors, that the applicable regulatory authorities could assert that payment of dividends or other payments is an unsafe or unsound banking practice. In the event that ACNB Bank is unable to pay dividends to ACNB, ACNB may not be able to pay dividends on its common stock. As of the date of this proxy statement/prospectus, ACNB has no such restrictions.

        ACNB common stock is listed on The NASDAQ Capital Market under the symbol "ACNB." The following table shows, for the indicated periods, the high and low sales prices per share for ACNB

common stock as reported on The NASDAQ Capital Market and dividends declared per share of ACNB common stock. These prices may include retail markups, markdowns, or commissions.

        On November 21, 2016, the last full trading day before the public announcement of the execution of the reorganization agreement, and on March 10, 2017, the latest practicable trading day before the printing of this document, the high, low and closing sales prices for ACNB common stock were as follows:

New Windsor

        As of March 17, 2017, there were 1,008,422 shares of New Windsor common stock outstanding which were held by approximately 453 holders of record. The number of stockholders does not reflect the number of individuals or institutional investors holding stock in nominee name through banks, brokerage firms, and others.

        New Windsor common stock is listed on the OTC Pink market place under the symbol "NWID." There is a limited public trading market for New Windsor common stock. The following table shows, for the indicated periods, the high and low sales prices per share for New Windsor common stock as

reported on the OTC Pink market place in each case as adjusted for the 5% stock dividend paid on February 13, 2015. These prices may include retail markups, markdowns, or commissions.

        New Windsor reinstated the payment of a cash dividend in the first quarter of 2015. There can be no assurance that future earnings will be sufficient to allow New Windsor to maintain or increase the amount of the cash dividend. New Windsor State Bank is the primary operating subsidiary of New Windsor and the source of earnings from which a dividend may be paid. Future dividend payments will depend largely upon the ability of New Windsor State Bank to declare and pay dividends to New Windsor. Payment of dividends on New Windsor common stock will therefore depend upon New Windsor State Bank's earnings, financial condition, and need for funds, as well as governmental policies and regulations applicable to New Windsor and New Windsor State Bank.

        Under Maryland law, dividends may only be paid out of retained earnings. State and federal bank regulatory agencies also have authority to prohibit a bank from paying dividends if such payment is deemed to be an unsafe or unsound practice, and the Federal Reserve Board has the same authority over bank holding companies. At December 31, 2016, New Windsor State Bank could pay dividends to New Windsor to the extent of its retained earnings so long as it maintained required capital ratios.

        The Federal Reserve Board has established requirements with respect to the maintenance of appropriate levels of capital by registered bank holding companies. Compliance with such standards, as presently in effect, or as they may be amended from time to time, could possibly limit the amount of dividends that New Windsor may pay in the future. In 1985, the Federal Reserve Board issued a policy statement on the payment of cash dividends by bank holding companies. In the statement, the Federal Reserve Board expressed its view that a holding company experiencing earnings weaknesses should not pay cash dividends exceeding its net income, or which could only be funded in ways that weaken the holding company's financial health, such as by borrowing. As a depository institution, the deposits of which are insured by the FDIC, the Bank may not pay dividends or distribute any of its capital assets while it remains in default on any assessment due the FDIC. New Windsor State Bank currently is not in default under any of its obligations to the FDIC.

        On November 21, 2016, the last full trading day before the public announcement of the execution of the reorganization agreement, and on                , 2017, the latest practicable trading day before the printing of this document, the high, low and closing sales prices for New Windsor common stock were as follows:

        Pursuant to the reorganization agreement, ACNB and New Windsor have agreed to coordinate with one another with respect to regular quarterly dividends to ensure that holders of New Windsor common stock do not receive two dividends, or fail to receive one dividend, for any quarter with respect to their shares of New Windsor common stock and any shares of ACNB common stock received in the merger.

Comparative Market Value

        The following table sets forth the market value per share of ACNB and New Windsor common stock and the equivalent market value per share of New Windsor common stock on November 21, 2016 (the last trading day prior to the date of the public announcement of the merger) and March 10, 2017 (the latest practicable trading day prior to the date of this document). The equivalent market value is based upon the exchange ratio of 1.10 shares of ACNB common stock multiplied by the closing sales price of ACNB common stock on the specified date.

Adjournment or Postponement Proposal (Page 148)

        You are being asked to approve a proposal to grant the New Windsor boards of director discretionary authority to adjourn or postpone the special meeting, if necessary, to solicit additional proxies for the merger proposal in the event there are insufficient votes to approve and adopt the reorganization agreement and the merger.

COMPARATIVE HISTORICAL AND PRO FORMA PER SHARE DATA

        Presented below for ACNB and New Windsor are comparative historical and unaudited pro forma equivalent per share financial data as of and for the year ended December 31, 2015, and as of and for the nine months ended September 30, 2016. The information presented below should be read together with the historical consolidated financial statements of ACNB and New Windsor, including the related notes. The New Windsor financial statements are included in this proxy statement/prospectus beginning at page F-1. The ACNB financial statements are incorporated by reference into this proxy statement/prospectus. See "Where You Can Find More Information" on page 149.

        The unaudited pro forma information gives effect to the merger as if the merger had been effective on December 31, 2015 or September 30, 2016 in the case of the book value data, and as if the merger had been effective as of January 1, 2015 in the case of the earnings per share and the cash dividends data. The unaudited pro forma data combines the historical results of New Windsor into ACNB's consolidated financial statements. While certain adjustments were made for the estimated impact of fair value adjustments and other acquisition-related activity, they are not indicative of what could have occurred had the acquisition taken place on January 1, 2015.

        The unaudited pro forma adjustments are based upon available information and certain assumptions that ACNB and New Windsor management believe are reasonable. The unaudited pro forma data, while helpful in illustrating the financial characteristics of the combined company under one set of assumptions, does not reflect the impact of factors that may result as a consequence of the merger or consider any potential impacts of current market conditions or the merger on revenues, expense efficiencies or asset dispositions, among other factors, nor the impact of possible business model changes. As a result, unaudited pro forma data are presented for illustrative purposes only and do not represent an attempt to predict or suggest future results. Upon completion of the merger, the operating results of New Windsor will be reflected in the consolidated financial statements of ACNB on a prospective basis.

        The pro forma combined book value per share of ACNB is based upon the pro forma combined common stockholders' equity for ACNB and New Windsor divided by the total pro forma common shares of the combined entity and reflects New Windsor shares at the exchange ratio of 1.10.

        The pro forma combined book value per share of ACNB is based upon the pro forma combined common stockholders' equity for ACNB and New Windsor divided by the total pro forma common shares of the combined entity and reflects New Windsor shares at the exchange ratio of 1.10.

SELECTED FINANCIAL AND OTHER DATA OF ACNB

        The following summary presents Selected Consolidated Financial Data of ACNB as of and for the periods indicated. The financial data as of and for the years ended December 31, 2015, 2014, 2013, 2012 and 2011 has been derived from ACNB's audited consolidated financial statements. The financial data as of and for the nine months ended September 30, 2016 and 2015 has been derived from ACNB's unaudited consolidated financial statements. The information as of and for the nine months ended September 30, 2016 and 2015 is unaudited and reflects only normal recurring adjustments that are, in the opinion of ACNB's management, necessary for a fair presentation of the result for the interim periods presented. The results of operations for the nine months ended September 30, 2016 are not necessarily indicative of the results to be achieved by ACNB for all of 2016 or for any other period.

SELECTED FINANCIAL AND OTHER DATA OF NEW WINDSOR

        The following summary presents Selected Consolidated Financial Data of New Windsor as of and for each of the five years ended December 31, 2015 (which has been derived from New Windsor's audited consolidated financial statement), and as of and for the nine months ended September 30, 2016 and 2015. You should read this table together with the historical consolidated financial information contained in New Windsor's consolidated financial statements and related notes which are included as part of this proxy statement/prospectus. Information for the nine month periods ended September 30, 2016 and 2015 is derived from New Windsor's unaudited interim financial statements and has been prepared on the same basis as its audited financial statements and includes, in the opinion of New Windsor's management, all adjustments, consisting of only normal recurring adjustments, necessary to present fairly the data for such period. The results of operations for the nine month period ended September 30, 2016 do not necessarily indicate the results which may be expected for any future period or for the full year.

UNAUDITED PRO FORMA COMBINED CONSOLIDATED FINANCIAL DATA

        The Unaudited Pro Forma Combined Condensed Consolidated Financial Information has been prepared using the acquisition method of accounting, giving effect to the merger. The Unaudited Pro Forma Combined Condensed Consolidated Statement of Financial Condition combines the historical information of ACNB and of New Windsor as of September 30, 2016 and assumes that the merger was completed on that date. The Unaudited Pro Forma Combined Condensed Consolidated Statements of Income combine the historical financial information of ACNB and of New Windsor and give effect to the merger as if it had been completed as of January 1, 2015. The Unaudited Pro Forma Combined Condensed Consolidated Financial Information is presented for illustrative purposes only and is not necessarily indicative of the results of income or financial condition had the merger been completed on the date described above, nor is it necessarily indicative of the results of income in future periods or the future financial condition and results of income of the combined entities. The financial information should be read in conjunction with the accompanying notes to the Unaudited Pro Forma Combined Condensed Consolidated Financial Information. Certain reclassifications have been made to New Windsor's historical financial information in order to conform to ACNB's presentation of financial information.

        The actual value of ACNB's common stock to be recorded as consideration in the merger will be based on the value as of the closing date of the merger. The proposed merger is targeted for completion in either late in the second or early third quarter of 2017. There can be no assurance that the merger will be completed as anticipated. For purposes of the Unaudited Pro Forma Combined Condensed Consolidated Financial Information, the fair value of ACNB's common stock to be issued in connection with the merger was based on ACNB's closing stock price of $30.15 as of January 12, 2017.

        The Unaudited Pro Forma Combined Condensed Consolidated Financial Information includes estimated adjustments, including adjustments to record New Windsor's assets and liabilities at their respective fair values, and represents ACNB's pro forma estimates based on available fair value information as of the date of the reorganization agreement.

        The pro forma adjustments are subject to change depending on changes in interest rates and the components of assets and liabilities and as additional information becomes available and additional analyses are performed. The final allocation of the purchase price for the merger will be determined after it is completed and after completion of thorough analyses to determine the fair value of New Windsor's tangible and identifiable intangible assets and liabilities as of the date the merger is completed. Increases or decreases in the estimated fair values of the net assets as compared with the information shown in the Unaudited Pro Forma Combined Condensed Consolidated Financial Information may change the amount of the purchase price allocated to goodwill and other assets and liabilities and may impact ACNB's statement of income due to adjustments in yield and/or amortization of the adjusted assets or liabilities. Any changes to New Windsor's stockholders' equity, including results of operations from September 30, 2016 through the date the merger is completed, will also change the purchase price allocation, which may include the recording of a lower or higher amount of goodwill. The final adjustments may be materially different from the unaudited pro forma adjustments presented herein.

        We anticipate that the merger will provide the combined company with financial benefits that include reduced operating expenses. The Unaudited Pro Forma Combined Condensed Consolidated Financial Information, while helpful in illustrating the financial characteristics of the combined company under one set of assumptions, does not reflect the benefits of expected cost savings or opportunities to earn additional revenue and, accordingly, does not attempt to predict or suggest future results. It also does not necessarily reflect what the historical results of the combined company would have been had our companies been combined during these periods.

        The Unaudited Pro Forma Combined Condensed Consolidated Financial Information has been derived from and should be read in conjunction with the historical consolidated financial statements and the related notes of ACNB and of New Windsor, which are incorporated by reference or appear elsewhere in this proxy statement/prospectus.

COMBINED CONDENSED CONSOLIDATED PRO FORMA STATEMENT OF FINANCIAL CONDITION (Unaudited)