 Marcus was well known for playing the role of the precocious child but also worked extensively as a voice actor for Hanna-Barbera throughout the 1980s.He started acting at five playing Shelby on "Sigmund and the Sea Monsters" (1973–1975).He also appeared on "The Nancy Walker Show" (1976), "Mary Hartman, Mary Hartman" (1976–77) (Marcus played child evangelist Jimmy Joe Jeeter, who died of electrocution in the bathtub), "Grandpa Goes to Washington" (1978), "The Bad News Bears" and "Goodtime Girls" (1980).