SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported):

August 17, 2016

United Bankshares, Inc.

(Exact name of registrant as specified in its charter)

300 United Center

500 Virginia Street, East

Charleston, West Virginia 25301

(Address of Principal Executive Offices)

(304) 424-8800

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On August 17, 2016, United Bankshares, Inc. (“United”) and its wholly-owned subsidiary, UBV Holding Company, LLC (“UBV”) entered into an Agreement and Plan of Reorganization (the “Agreement”) with Cardinal Financial Corporation (“Cardinal”), a Virginia corporation headquartered in Tysons Corner, Virginia.

In accordance with the Agreement, Cardinal shall merge with and into UBV (the “Merger”). At which time, Cardinal shall cease to exist and UBV shall survive and continue to exist as a Virginia limited liability company. United may at any time prior to the effective time of the Merger change the method of effecting the combination with Cardinal subject to certain conditions contained in the Agreement.

The Agreement provides that upon consummation of the Merger, each outstanding share of common stock of Cardinal will be converted into the right to receive 0.71 shares of United common stock, par value $2.50 per share.

Pursuant to the Agreement, at the effective time of the Merger, each outstanding Cardinal stock option granted under a Cardinal stock plan, whether vested or unvested as of the date of the Agreement, shall vest and convert into an option to acquire United common stock adjusted based on the 0.71 exchange ratio. Also, at the effective time of the Merger, each restricted stock award granted under a Cardinal stock plan that is unvested or contingent and outstanding immediately prior to the effective time of the Merger shall fully vest and be converted into the right to receive shares of United common stock based on the 0.71 exchange ratio.

At the effective time of the Merger, Cardinal Bank, a wholly-owned subsidiary of Cardinal, will merge with and into United Bank, a wholly-owned indirect subsidiary of United (the “Bank Merger”). United Bank will survive the Bank Merger and continue to exist as a Virginia banking corporation.

Consummation of the Merger is subject to approval of the shareholders of United and Cardinal, the receipt of all required regulatory approvals, as well as other customary conditions.

The foregoing description of the Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Agreement, a copy of which is attached as Exhibit 2.1 to this Current Report on Form 8-K and is incorporated herein by reference. The Agreement has been attached as an exhibit to provide investors and security holders with information regarding its terms. It is not intended to provide any other financial information about United or its subsidiaries or affiliates. The representations, warranties and covenants contained in the Agreement were made only for purposes of that agreement and as of specific dates, are solely for the benefit of the parties to the Agreement, may be subject to limitations agreed upon by the parties, including being qualified by confidential disclosures made for the purposes of allocating contractual risk between the parties to the Agreement instead of establishing these matters as facts, and may be subject to standards of materiality applicable to the parties that differ from those applicable to investors. Investors should not rely on the representations, warranties, or covenants or any description thereof as characterizations of the actual state of facts or condition of United or any of its subsidiaries or affiliates. Moreover, information concerning the subject matter of the representations, warranties, and covenants may change after the date of the Agreement, which subsequent information may or may not be fully reflected in public disclosures by United.

On August 18, 2016, United and Cardinal issued a joint press release and United provided an investor presentation to interested parties concerning the acquisition of Cardinal. Copies of the press release and investor presentation are attached hereto as Exhibits 99.1 and 99.2, and are being furnished to the Securities and Exchange Commission and shall not be deemed “filed” for any purpose.

Forward-Looking Statements

This Form 8-K, the press release and investor presentation contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. These forward-looking statements are intended to be covered by the safe harbor provisions for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995. These forward-looking statements include, but are not limited to, statements about (i) the benefits of a merger (the “Merger”) between Cardinal Financial Corporation (“Cardinal”) and United Bankshares, Inc. (“United”), including future financial and operating results, cost savings enhancements to revenue and accretion to reported earnings that may be realized from the Merger; (ii) United’s and Cardinal’s plans, objectives, expectations and intentions and other statements contained in this press release that are not historical facts; and (iii) other statements identified by words such as “expects,” “anticipates,” “intends,” “plans,” “believes,” “seeks,” “estimates,” “targets,” “projects,” or words of similar meaning generally intended to identify forward-looking statements. These forward-looking statements are based upon the current beliefs and expectations of the respective managements of United and Cardinal and are inherently subject to significant business, economic and competitive uncertainties and contingencies, many of which are beyond the control of United and Cardinal. In addition, these forward-looking statements are subject to assumptions with respect to future business strategies and decisions that are subject to change. Actual results may differ materially from the anticipated results discussed in these forward-looking statements because of possible uncertainties.

The following factors, among others, could cause actual results to differ materially from the anticipated results or other expectations expressed in the forward-looking statements: (1) the businesses of United and Cardinal may not be combined successfully, or such combination may take longer, be more difficult, time-consuming or costly to accomplish than expected; (2) the expected growth opportunities or cost savings from the Merger may not be fully realized or may take longer to realize than expected; (3) deposit attrition, operating costs, customer losses and business disruption following the Merger, including adverse effects on relationships with employees, may be greater than expected; (4) the regulatory approvals required for the Merger may not be obtained on the proposed terms or on the

anticipated schedule; (5) the stockholders of United and Cardinal may fail to approve the Merger; (6) legislative or regulatory changes, including changes in accounting standards, may adversely affect the businesses in which United and Cardinal are engaged; (7) changes in the interest rate environment may compress margins and adversely affect net interest income; (8) results may be adversely affected by continued diversification of assets and adverse changes to credit quality; (9) competition from other financial services companies in United’s and Cardinal’s markets could adversely affect operations; and (10) an economic slowdown could adversely affect credit quality and loan originations. Additional factors, that could cause actual results to differ materially from those expressed in the forward-looking statements are discussed in Cardinal’s and United’s reports (such as Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K) filed with the Securities and Exchange Commission and available on the SEC’s Internet site (http://www.sec.gov).

United and Cardinal caution that the foregoing list of factors is not exclusive. All subsequent written and oral forward-looking statements concerning the proposed transaction or other matters attributable to United or Cardinal or any person acting on their behalf are expressly qualified in their entirety by the cautionary statements above. United and Cardinal do not undertake any obligation to update any forward-looking statement to reflect circumstances or events that occur after the date the forward-looking statements are made.

Additional Information About the Merger and Where to Find It

Shareholders of Cardinal and other investors are urged to read the proxy statement/prospectus that will be included in the registration statement on Form S-4 that United will file with the Securities and Exchange Commission in connection with the proposed merger because it will contain important information about United, Cardinal, the merger, the persons soliciting proxies in the merger and their interests in the merger and related matters. Investors will be able to obtain all documents filed with the SEC by United free of charge at the SEC’s Internet site (http://www.sec.gov). In addition, documents filed with the SEC by United will be available free of charge from the Corporate Secretary of United Bankshares, Inc., 514 Market Street, Parkersburg, West Virginia 26101 telephone (304) 424-8800. The proxy statement/prospectus (when it is available) and the other documents may also be obtained for free by accessing United’s website at www.ubsi-inc.com under the tab “Investor Relations” and then under the heading “SEC Filings” or by accessing Cardinal’s website at www.cardinalbank.com under the tab “About Us” and then under the heading “Investor Relations” and “SEC Filings”. You are urged to read the proxy statement/prospectus carefully before making a decision concerning the merger. You are urged to read the proxy statement/prospectus carefully before making a decision concerning the merger.

Participants in the Transactions

United, Cardinal and their respective directors, executive officers and certain other members of management and employees may be deemed “participants” in the solicitation of proxies from Cardinal’s shareholders in favor of the merger with United. Information regarding the persons who may, under the rules of the SEC, be considered participants in the solicitation of the Cardinal shareholders in connection with the proposed merger will be set forth in the proxy statement/prospectus when it is filed with the SEC.

You can find information about the executive officers and directors of United in its Annual Report on Form 10-K for the year ended December 31, 2015 and in its definitive proxy statement filed with the SEC on April 1, 2016. You can find information about Cardinal’s executive officers and directors in its Annual Report on Form 10-K for the year ended December 31, 2015 and in its definitive proxy statement filed with the SEC on March 24, 2016. You can obtain free copies of these documents from United or Cardinal using the contact information above.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.