Atara Biotherapeutics, Inc.

PROXY CARD

ATARA BIOTHERAPEUTICS, INC SCAN TO VIEW MATERIALS & VOTE VOTE BY INTERNET 2380 CONEJO SPECTRUM STREET, SUITE 200 Before The Meeting - Go to www.proxyvote.com or scan the QR Barcode above THOUSAND OAKS, CA 91320 Use the Internet to transmit your voting instructions and for electronic delivery of information up until 11:59 p.m. Eastern Time on June 9, 2024. Have your proxy card in hand when you access the web site and follow the instructions to obtain your records and to create an electronic voting instruction form. During The Meeting - Go towww.virtualshareholdermeeting.com/ATRA2024 You may attend the meeting via the Internet and vote during the meeting. Have the information that is printed in the box marked by the arrow available and follow the instructions. VOTE BY PHONE - 1-800-690-6903 Use any touch-tone telephone to transmit your voting instructions up until 11:59 p.m. Eastern Time on June 9, 2024. Have your proxy card in hand when you call and then follow the instructions. VOTE BY MAIL Mark, sign and date your proxy card and return it in the postage-paid envelope we have provided or return it to Vote Processing, c/o Broadridge, 51 Mercedes Way, Edgewood, NY 11717. TO VOTE, MARK BLOCKS BELOW IN BLUE OR BLACK INK AS FOLLOWS: V46741-P08093 KEEP THIS PORTION FOR YOUR RECORDS THIS PROXY CARD IS VALID ONLY WHEN SIGNED AND DATED. DETACH AND RETURN THIS PORTION ONLY ATARA BIOTHERAPEUTICS, INC. The Board of Directors recommends you vote FOR the following: 1. Election of Directors Nominees: For Withhold 1a. William K. Heiden 1b. Ameet Mallik The Board of Directors recommends you vote FOR the following proposals: For Against Abstain 2. To approve, on an advisory basis, the compensation of the Company's named executive officers, as disclosed in the Proxy Statement. 3. To ratify the selection of Deloitte & Touche LLP as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2024 4. To approve the Company's 2024 Equity Incentive Plan. 5. To approve an automatic annual increase to the Company's 2024 Equity Incentive Plan. 6. To approve an increase in the number of shares of common stock available for issuance under the Company's 2014 Employee Stock Purchase Plan. 7. To approve an amendment to the Company's amended and restated certificate of incorporation to effect a reverse stock split of the Company's common stock at a ratio ranging from any whole number between 1-for-4 and 1-for-30, as determined by the Board of Directors in its discretion, subject to the Board of Directors' authority to abandon such amendments (the "Charter Amendment Proposal").8. To approve the adjournment of the Annual Meeting to a later date or dates, if necessary, to permit further solicitation and vote of proxies in the event there are not sufficient votes in favor of the Charter Amendment Proposal. NOTE: Such other matters that may properly come before the meeting or any adjournment or postponement thereof will be voted on by the proxy holders in their discretion. Please sign exactly as your name(s) appear(s) hereon. When signing as attorney, executor, administrator, or other fiduciary, please give full title as such. Joint owners should each sign personally. All holders must sign. If a corporation or partnership, please sign in full corporate or partnership name by authorized officer. Signature [PLEASE SIGN WITHIN BOX] Date Signature (Joint Owners) Date

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting: The Notice and Proxy Statement and 2023 Annual Report on Form 10-K are available atwww.proxyvote.com. ATARA BIOTHERAPEUTICS, INC. Annual Meeting of Stockholders June 10, 2024 9:00 AM Pacific Time This proxy is solicited by the Board of Directors The undersigned stockholder hereby appoints Pascal Touchon and Eric Hyllengren, or either of them, as proxies, each with the power to appoint his substitute, and hereby authorizes them to represent and to vote, as designated on the reverse side of this ballot, all of the shares of common stock of ATARA BIOTHERAPEUTICS, INC. that the stockholder is entitled to vote at the Annual Meeting of Stockholders to be held at 9:00 AM Pacific Time on June 10, 2024 virtually at www.virtualshareholdermeeting.com/ATRA2024, and any adjournment or postponement thereof. This proxy, when properly executed, will be voted in the manner directed herein. If no such direction is made, this proxy will be voted FOR all nominees listed in Proposal 1 and FOR Proposals 2, 3, 4, 5, 6, 7, and 8 in accordance with the recommendations of the Board of Directors, and in the discretion of the proxy holders upon any other business as may properly come before the Annual Meeting of Stockholders or any adjournment or postponement thereof. Continued and to be signed on reverse side