Martini was born in Denver, Colorado. It was at his suggestion that Sly and the Family Stone came about, and he became one of its key figures. He performed with the band from its inception in 1967 to its demise in 1975. In 1975, he appeared on Sly Stone's first solo album, High on You.
In 2004–2005, he helped form the tribute band FamilyStoneExperience bringing together several Family Stone alumni, including Greg Errico, Cynthia Robinson, Dawn Silva and Gail Muldrow amongst others.
In 2006, after a stint with Prince's touring band (which also featured Sly alumnus Larry Graham), he reunited with the original Family Stone.