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