Clark was born Richard Mardirosian in Buffalo, New York, the son of Afro (née Karahos) and Matthew Mardirosian. He is of Armenian heritage. His brother, Tom Mardirosian, is also an actor. Clark made numerous appearances in film and television, particularly in the 1970s and 1980s. Two of his best-known characters were John Doe #6 on the NBC drama St. Elsewhere, a likeable and humorous psychiatric patient, and Mr. Herd, a patient of Psychologist Bob Hartley on the CBS situation comedy The Bob Newhart Show. He played Mr. Belding in the original pilot of the series Good Morning Miss Bliss but was subsequently replaced by Dennis Haskins.
Clark was a regular on the 1975 situation comedy Karen. In 1977, he appeared in an episode of the ABC crime drama The Feather and Father Gang.
