Two 15-year-old girls inside the vehicle were also injured, one critically, during the late Saturday night accident, police said. Garrison, 26, had minor injuries.
The vehicle jumped a curb and hit a tree. Authorities were trying to determine who drove the SUV, which was registered to Garrison.
The boy was taken to Cedars-Sinai Medical Center where he died. His name was not released. The conditions of the girls were not immediately released.
Garrison, a Dallas native, plays "Tweener" on FOX's "Prison Break." He has worked mainly in television, but appears in the upcoming film "Shooter," which is due out next year.
Garrison's publicist, Catherine Olim, did not immediately return a phone call Sunday.