Born on November 8, 1961 in Hollywood, California, Leif grew up in a world of showbiz and got his first taste of acting in the blockbuster film, Bob & Carol & Ted & Alice (1969) playing the son of Elliott Gould and Dyan Cannon, though he was uncredited. Then he was a frequent guest in TV shows such as Nanny and the Professor (1970), Family Affair (1966), Cannon (1971), Gunsmoke (1955) and The Odd Couple (1970) as well as receiving brief exposure in Walking Tall (1973) and Peopletoys (1974), both films also starring his sister, Dawn Lyn. He then played Endy Karras in the TV series Three for the Road (1975), also starring Alex Rocco and Vincent Van Patten, and by then he started getting exposure on teen magazines and fan mail, despite the show's short run. He still pursued his acting career by starring in some westerns like Diamante Lobo (1976), Kid Vengeance (1976) and Peter Lundy and the Medicine Hat Stallion (1977).

Garrett was offered a recording contract by Atlantic Records. He released two oldie cover singles "Surfin' USA" and "Runaround Sue" on his self-titled debut album, but wasn't really happy with this style of music. After switching to the Scotti Brothers he recorded another album called "Feel the Need" which had a top 10 disco-flavoured hit called, "I Was Made for Dancin'". He released three more albums but mostly stopped recording music in the early 1980s.

He continued in films and is well remembered for his small supporting role in the blockbuster film The Outsiders (1983), which starred Matt Dillon, Patrick Swayze, Rob Lowe, Emilio Estevez and Tom Cruise, but his films after that were less successful. However, he did return to music by playing in a band called "Godspeed" in the late 1990s, this time playing his own music.