Garrison, the former "Prison Break" star, told a Los Angeles judge, "I am guilty," E! News reported.
Garrison also pleaded guilty to felony DUI with bodily injury, and to a misdemeanor charge of providing alcohol to minors. The charges stem from a December crash that killed 17-year-old high school student and injured two 15-year-old girls.
He also admitted his blood alcohol count was higher than 0.15 when the accident occurred.
Garrison is scheduled to be sentenced Aug. 2. He could be sentenced to a maximum of 68 months in prison.
The incident occurred Dec. 2 when Garrison reportedly met a group of teenagers at a supermarket and agreed to accompany them to a party. As the actor drove through Beverly Hills, his vehicle jumped a curb and hit a tree, authorities said.
A police investigation revealed Garrison had a blood-alcohol level of 0.2 -- more than twice the legal limit in California -- at the time of the crash and tested positive for cocaine.