McManus was born in Hamilton, Lanarkshire, Scotland, on 21 February 1935. Mark was moved to Hillingdon, Uxbridge, in London when he was three years old until he moved again at the age of 16 to Australia where he performed in amateur theatre groups which led him to become a professional actor. He appeared in the popular children's TV series Skippy the Bush Kangaroo, and made a guest appearance in the long-running Australian police drama Homicide. He also starred in Tim Burstall's unsuccessful but historically important feature film 2000 Weeks (1969), which was the first full-length Australian-produced feature made in Australia since Charles Chauvel's Jedda in 1954.