Garrison, 28, is best known for his work on the drama series "Prison Break."
He was arrested in 2006 in connection with a wreck that killed a 17-year-old passenger and injured two 15-year-old girls in his car.
E! News said Garrison was greeted by family and friends outside of the California Correctional Institute in rural Tehachapi.
"He's done his time and now he's looking forward to trying to move on with his life," a source close to the actor told E!
Garrison is expected to soon check into an in-patient rehabilitation center to shorten his probation period, sources said.