MUSTANG BIO, INC.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of

the Securities Exchange Act of 1934

Check the appropriate box:

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

MUSTANG BIO, INC.

95 Sawyer Road, Suite 110

Waltham, Massachusetts 02453

Dear Stockholder:

You are cordially invited to the Annual Meeting of Stockholders (the “Annual Meeting”) of Mustang Bio, Inc. (“Mustang” or the “Company”), to be held virtually at 9:00 a.m. Eastern Time, on Monday, December 22, 2025. The Annual Meeting can be accessed by visiting www.virtualshareholdermeeting.com/MBIO2025, where you will be able to listen to the meeting live, submit questions and vote online. At the Annual Meeting, the stockholders will be asked to (i) elect seven directors for a term of one year, (ii) ratify the appointment of KPMG LLP as our independent registered public accounting firm for the year ending December 31, 2025, (iii) approve an amendment to the Company’s 2019 Employee Stock Purchase Plan to increase the number of shares issuable by 250,000 shares and increase the number of shares subject to a Purchase Right thereunder to 10,000, (iv)approve an amendment to the Company’s 2016 Incentive Plan to increase the number of shares issuable by 2,500,000 shares and (v) transact any other business that may properly come before the Annual Meeting or any adjournment of the Annual Meeting. You will also have the opportunity to ask questions at the meeting.

Your vote is important. It is important that your stock be represented at the meeting regardless of the number of shares you hold. To be sure your vote counts and assure a quorum, please vote by mobile device or over the Internet, or if you received proxy materials by mail, vote, sign, date and return the proxy card accompanying the printed proxy materials, as soon as possible, regardless of whether you plan to virtually attend the meeting; or if you hold your shares through a bank, brokerage firm or other nominee, please follow the instructions for voting provided by your bank, brokerage firm or other nominee, regardless of whether you plan to attend the meeting virtually. If you virtually attend the Annual Meeting and wish to vote virtually, you may revoke your proxy at the meeting.

If you have any questions about the proxy statement or the accompanying 2024 Annual Report, please contact us at info@mustangbio.com.

We look forward to seeing you at the Annual Meeting.

December 3, 2025

Waltham, Massachusetts

MUSTANG BIO, INC.

95 Sawyer Road, Suite 110

Waltham, Massachusetts 02453

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

The Annual Meeting of Stockholders of Mustang Bio, Inc. will be held virtually on December 22, 2025, at 9:00 a.m., Eastern Time. At the Annual Meeting, stockholders will consider and act on the following items:

Only those stockholders of record as of the close of business on November 18, 2025, are entitled to vote at the Annual Meeting or any postponements or adjournments thereof. A complete list of stockholders entitled to vote at the Annual Meeting will be available for your inspection 10 days preceding our Annual Meeting, at our offices located at 95 Sawyer Road, Suite 110, Waltham, Massachusetts 02453, between the hours of 10:00 a.m. and 5:00 p.m., ET, each business day, or if we determine that a physical in-person inspection is not practicable, such list of stockholders may be made available electronically, upon request. You or your proxyholder may participate and vote by visiting www.virtualshareholdermeeting.com/MBIO2025 and using your 16-digit control number.

Please note the technical requirements for virtual attendance at the Annual Meeting, as described in the enclosed Proxy Statement under the heading “Questions and Answers.”

On January 15, 2025, we filed an amendment (the “Reverse Split Amendment”) to our Amended and Restated Certificate of Incorporation (our “Certificate of Incorporation”) with the Secretary of State of the State of Delaware to effect a 1-for-50 reverse stock split of our common stock (the “Reverse Stock Split”). As a result of the Reverse Stock Split, every 50 shares of common stock outstanding immediately prior to effectiveness of the Reverse Stock Split were combined and converted into one share of common stock without any change in the par value per share. The Reverse Stock Split became effective on January 15, 2025. No fractional shares were issued in connection with the Reverse Stock Split. Stockholders who would have otherwise been entitled to a fraction of one share of common stock as a result of the Reverse Stock Split instead received a proportional cash payment. All share and per share information in this proxy statement has been retroactively adjusted to give effect to the Reverse Stock Split, unless otherwise indicated.

IMPORTANT NOTICE REGARDING AVAILABILITY OF PROXY MATERIALS. THIS PROXY STATEMENT AND THE ENCLOSED PROXY CARD ARE BEING MAILED TO OUR STOCKHOLDERS ON OR ABOUT DECEMBER 3, 2025. In accordance with the rules of the Securities and Exchange Commission (the “SEC”), we are advising our stockholders of the availability on the internet of our proxy materials related to our forthcoming Annual Meeting. Because we have elected to utilize the“full set delivery”** option, we are delivering to all stockholders** paper copies of all of the proxy materials, as well as providing access to those proxy materials on a publicly accessible website. This Notice of Annual Meeting and Proxy Statement are available to holders of our common stock at www.proxyvote.comand on our corporate websitewww.mustangbio.com**.**

YOUR VOTE IS IMPORTANT!

Submitting your proxy does not affect your right to vote virtually if you decide to virtually attend the Annual Meeting. You are urged to submit your proxy as soon as possible, regardless of whether or not you expect to virtually attend the Annual Meeting. You may revoke your proxy at any time before it is voted at the Annual Meeting by (i) delivering written notice to our Corporate Secretary at our address above,(ii) submitting a later dated proxy card, (iii) voting again via the internetor (iv) attending the Annual Meeting and voting virtually. No revocation under (i) or (ii) will be effective unless written notice or the proxy card is received by our Corporate Secretary at or before the Annual Meeting.

When you submit your proxy, you authorize Manuel Litchman, M.D. and/or Peter Carney or their designees to vote your shares at the Annual Meeting and on any adjournments of the Annual Meeting in accordance with your instructions.

December 3, 2025

Waltham, Massachusetts