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 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).
He later had regular roles as a voice actor on Shirt Tales (1982–1984), Space Stars (1981), Saturday Supercade as the voice of Dexter on Space Ace (1984), Challenge of the GoBots (1984), The Puppy's Further Adventures (1983), The Reluctant Dragon (1981), Cabbage Patch Kids: First Christmas (1984), Banjo the Woodpile Cat (1979), CBS Library (1980–1982) and The Get Along Gang (1984–86).
Marcus has also made several TV guest appearances, including Trapper John, M.D., Happy Days, WKRP in Cincinnati, What's Happening!
Marcus also appeared in the feature films Freaky Friday (1976) and The Pinballs (1977), as well as in many TV movies, including The Stableboy's Christmas (1978) and Goldie and the Boxer (1979).