X0306 4 2021-05-17 0 0001595974 MAGNITE, INC. MGNI 0001100612 BARRETT MICHAEL G. C/O MAGNITE, INC. 6080 CENTER DRIVE, 4TH FLOOR LOS ANGELES CA 90045 1 1 0 0 CEO Common Stock 2021-05-17 4 S 0 105138 25.98 D 1423770 D Represents the non-discretionary sale of shares on behalf of the Reporting Person pursuant to an arrangement mandated by the Issuer to cover the tax withholding obligations associated with the vesting of restricted stock units. Adjusted to reflect the acquisition of 2,230 shares under the Magnite, Inc. 2014 Employee Stock Purchase Plan on May 17, 2021. /s/ Aaron Saltz, attorney-in-fact 2021-05-19