Sangster originally worked as a production assistant at Hammer Studios, as well as assistant director, second unit director and production manager. After Hammer Films Productions' success with The Quatermass Xperiment, Sangster was approached to write The Curse of Frankenstein, to which he said, "I'm not a writer. I'm a production manager." According to Sangster, Hammer Films' response was, "Well, you come up with a couple of ideas and if we like it, we'll pay you. If we don't like it, we won't pay you. You're being paid as a production manager, so you can't complain." He later turned to direction with The Horror of Frankenstein and Lust for a Vampire (both 1970) for the studio, but with far less success. His third (and last) film as director was 1972's Fear in the Night, which resurrected the psychological woman-in-peril thriller Sangster had begun with his script for Taste of Fear in 1961. All three of these films featured actor Ralph Bates, one of Hammer's best-known actors of the latter period of the company.
He died on August 19, 2011. He is survived by his third wife, the actress Mary Peach and by a son from an earlier marriage, Mark James Sangster  and two grandchildren, Claire and Ian Sangster.