Garnett was born in Auckland, New Zealand. When Garnett was 11, her family moved to Canada, and she made her public singing debut in 1960, while at the same time pursuing an acting career making guest appearances on television shows such as 77 Sunset Strip. She made her "New York nightclub debut" in 1963, and was signed that same year to RCA Records. In the fall of 1964, Garnett scored a #4 pop hit with her original composition "We'll Sing in the Sunshine" (also #1 on Billboard's Adult Contemporary singles chart for seven weeks and a Top 50 country hit), and recorded her debut album, My Kind of Folk Songs, for RCA Victor. Riding the success of "We'll Sing in the Sunshine," which won a 1965 Grammy for Best Folk Recording, Garnett continued to record through the rest of the 1960s with her backing band the Gentle Reign. However, her follow-up to "We'll Sing In The Sunshine," "Lovin' Place," was her only other single to chart in America. Garnett also twice appeared on ABC's Shindig! at the height of her singing fame in the mid 1960s.

Garnett delivered a notable performance in the Rankin-Bass feature Mad Monster Party in the late 1960s, with the memorable tunes "Our Time to Shine" and "Never Was a Love Like Mine." At this period she had begun to be more influenced by the counter-culture and had embraced psychedelic themes to some extent. In the late 1960s she recorded several albums of psychedelic-inflected music with a band called the Gentle Reign.

Although Garnett had retired from the music business by the 1980s, she continued appearing in feature films (such as 2002's My Big Fat Greek Wedding) and on television shows, usually in supporting roles. In subsequent years, she also branched out into journalism, writing essays, columns, and book reviews for various newspapers and magazines. She also wrote and performed two one-person theater pieces, Gale Garnett & Company and Life After Latex.

Garnett published her first novel, a romance titled Visible Amazement, in 1999. She followed Visible Amazement up with Transient Dancing in 2003, the novella Room Tone in 2007, and Savage Adoration, her latest release, in 2009.