Hemmings was a professional singer by the age of 9 and an exhibited painter at 15. He entered films in 1950 and is best known for his first starring role, as the photographer in Michelangelo Antonioni's classic portrait of "swinging London," "Blow-Up" (1966). Wide-eyed and interesting-looking rather than conventionally handsome, Hemmings was a fashionably offbeat leading man for a while thereafter in "Camelot" (1967), "Barbarella" (1968) and "The Charge of the Light Brigade" (1968), but many of his 1970s and early 80s credits were in little-seen if sometimes interesting fare. In 1967 Hemmings co-founded the Hemdale Corporation with his business manager John Daly but he left the company in 1970.
Hemmings has also directed several films, including "Just a Gigolo" (1979), and in the 80s he launched a second career as a TV director for series including "Magnum P.I.", "Airwolf", "The New Mike Hammer", "Hawaiian Heat", "The Last Precinct", "Raven" and "In the Heat of the Night". Although he never completely gave up acting, most of Hemmings' histrionic activity was limited to the small screen as well; in 1989, though, he returned to feature films after a six-year absence to play Uncle Henry in Ken Russell's "The Rainbow". He is divorced from actress Gayle Hunnicutt.