Marcus studied at the Berklee School of Music in Boston and appeared in the groups of Stan Kenton, Gary Burton, Woody Herman, and Herbie Mann in the 1960s. In 1968, he recorded Tomorrow Never Knows, a then-novel mix of avant-garde jazz and psychedelic rock. He made two similar albums, Count's Rock Band and The Lord's Prayer, the following year.
Marcus returned to big-band jazz in 1975, when he joined Rich's big band. He played with Rich until the drummer's death in 1987, and later appeared in a big band and a smaller group devoted to Rich's music.