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. He subsequently pleaded guilty to vehicular manslaughter and drunk driving.
Garrison reportedly was greeted by family and friends outside of the California Correctional Institute in Tehachapi when he was released Wednesday.
E! News said Garrison enjoyed a brief celebration at Jessica Simpson's parents' house in Encino that afternoon, and then headed to the Los Angeles-area Tarzana Treatment Center to begin a five-month, in-patient program.
"I'm blessed to be alive," Garrison told E! "I'm just worried about getting through treatment right now. ... I'm looking forward to getting on with my life and doing the right thing and helping people."
Garrison lived with the Simpson family for a while and Jessica and Ashlee's dad Joe was his youth minister.
Jessica attended the welcome-home party for Garrison, E! said, but Ashlee was not spotted there.
McPhee, Cokas 'working on their marriage' after affair
Astronomers offer more expansive view of universe