Garrett, 49, pleaded no contest to the charge last year and entered a Proposition 36 drug-treatment programme, which allows defendants to receive treatment in lieu of a prison sentence.
The singer-actor was arrested on 1 February last year at a subway station in LA, after consenting to a search by sheriff's deputies.
He had previously been arrested at the Pershing Square Red Line station in January 2006 for possession of heroin.
The former Three For The Road star was sentenced in May 2006 to 90 days in a county jail and three years’ probation in connection with two drug-related cases, after opting out of a Drug Court programme.
According to the criminal complaint, Garrett also had a March 2001 conviction in a drug-related case. But that case was dismissed after he showed that he had completed a drug programme.
In the 1970s, Garrett frequently appeared in teen magazine Tiger Beat, and was also the subject of a VH-1 Behind The Music special that traced his rise from child star on shows such as The FBI and Gunsmoke to the decline of his musical career in the early 1980s.