Clark was born in Washington D.C., on November 25, 1936, the son of Theresa (née Castello), a teacher, and Frederick William Clark, a carpenter. Clark grew up in Conyers, Georgia. After serving in the military, he attended college at George Washington University, but later dropped out. After working at various jobs, he joined a local D.C. theatre group. He later became a member of New York's Living Theatre company and worked off-Broadway and in community theatre in the late 1950s.