Brennan was a patient at a rehabilitation facility when he was found unconscious a few days ago. He died at a hospital in Newport, Calif.
The Laguna Beach native starred as the Rainbow Warriors' signal-caller from 2005 to 2007, a run that included a memorable 12-0 season as a senior.
Brennan was a Heisman Trophy finalist in 2007 following his record-setting campaign. He's the school's all-time leader in passing touchdowns, and Brennan established the top single-season team marks with 5,549 yards and 58 scores through the air.
He also held the NCAA record for the most single-season touchdown passes before Joe Burrow eclipsed his total in 2019 with LSU.
Washington selected Brennan with its sixth-round pick in 2008. He spent two years with the club and one with the then-Oakland Raiders. Brennan also played in the CFL and Arena Football League later in his career.