[1] Sinclair was a juvenile player turned film editor who retained his celebrity in his native New Zealand long after the end of his Hollywood acting career.

Sinclair's feature credits include William Wellman's The Light That Failed, Tower of London, Alexander Korda's That Hamilton Woman, Raoul Walsh's Desperate Journey, and Thoroughbreds Don't Cry with Mickey Rooney and Judy Garland.

After that, he resumed work in 1955, and began a long, fruitful collaboration with producer-director Roger Corman, which led to a busy career in low-budget independent filmmaking.

He also edited a number of films by another low-rent auteur with big ideas, Bert I. Gordon: The Amazing Colossal Man, Invasion of the Saucer Men, War of the Colossal Beast, Attack of the Puppet People, and The Spider.

Sinclair died from respiratory failure at the Motion Picture & Television County Home on 22 November 1992.