Weitz was born Mark Stephen Weitz in Brooklyn, New York, in 1945 and at 6 months old moved to California. He took up playing piano and organ at age 8 and at age 20 joined a rock group called Thee Sixpence as one of the singers and the organist. Three or four years older than everyone else, he had more definite musical ideas than his bandmates, as well as a more mature and professional outlook on music, which served them well the next four years. He also found something of a kindred musical spirit in Ed King, the band's lead guitarist.
Weitz was an able composer, and for the group's 1967 single on the all American label he turned in three songs: "The Birdman of Alkatrash", "Heart full of Rain",(written just by him) and "Incense and Peppermints", a collaboration with Ed King. The latter, turned over to another composer by the record's producer to write the lyrics, became a #1 national hit for the group, newly christened The Strawberry Alarm Clock. For the next three years, Weitz rode a whirlwind of dizzying success and frustrating attempts at a follow-up, though he did prove the single wasn't a fluke and wasn't to the credit of the lyricist by creating a top 30 hit called "Tomorrow" (Lyrics written by Weitz and music by Weitz and King). Weitz played some memorable keyboard parts on their first three albums, and on S.A.C.'s album Good Morning Starshine he showed himself in collaboration with King to be an able first-time producer.
Later life, Strawberry Alarm Clock, Billy Corgan