Born in Guildford, Surrey, Hemmings started his career as a boy soprano, appearing in several works by Benjamin Britten, who formed a close friendship with him at this time. Most notably, he created the role of Miles in the opera Turn of the Screw.

Hemmings' intimate, yet innocent, relationship with Britten is described in John Bridcut's Britten's Children. Although many commentators identified Britten's relationship with Hemmings as based on an infatuation, throughout his life Hemmings maintained categorically that Britten's conduct with him was beyond reproach at all times.

Hemmings then moved on to an acting and directing career in the cinema. He made his first film appearance in 1954, but it was in the mid-sixties that he first became well known as a pin-up and film star. Antonioni, who detested the "Method" way of acting, sought to find a fresh young face for the lead in his next production.

It was then that he found Hemmings, at the time acting in small stage theatre in London. Following Blowup, Hemmings appeared in a string of major British films, including Camelot (1967), The Charge of the Light Brigade (1968) and Alfred the Great (1969) (in which he played the title role). In keeping with his standing as a 1960s icon, he also appeared in Barbarella.

Around 1967 Hemmings was also briefly considered for the role of Alex in a planned film version of Anthony Burgess' controversial novel A Clockwork Orange which was to be based on a screen treatment by satirist Terry Southern and British photographer Michael Cooper.

Cooper and The Rolling Stones were reportedly upset by the move and it was decided to return to the original plan in which Mick Jagger would play Alex, with the rest of The Rolling Stones as his droog gang, but the production was shelved after Britain's chief censor, the Lord Chamberlain, indicated that he would not permit it to be made.

Another (Italian) cult movie in which Hemmings appeared was the 1975 thriller Profondo Rosso (also known as Deep Red or The Hatchet Murders) directed by Dario Argento.

In 1978 Hemmings directed David Bowie and Marlene Dietrich in Schöner Gigolo, armer Gigolo (also known as Just a Gigolo). The film was poorly received, Bowie describing it as "my 32 Elvis Presley movies rolled into one". Hemmings directed a film version of James Herbert's novel The Survivor, starring Robert Powell and Jenny Agutter, in 1981.

Throughout the 1980s he also worked extensively as a director on television programmes including Magnum, P.I. (in which he also played characters in several episodes), The A-Team and Airwolf, in which he also played the role of Doctor Charles Henry Moffet, Airwolf's twisted creator, in the pilot and the second season episode "Moffett's Ghost" (a typographical error by the studio's titles unit). He once joked, "People thought I was dead. But I wasn't. I was just directing The A-Team." In 1984 he directed the puzzle contest video Money Hunt: The Mystery of the Missing Link.

Hemmings played a vindictive cop in the 1980 movie Beyond Reasonable Doubt about Arthur Allan Thomas, a New Zealand farmer jailed for the murder of Harvey and Jeanette Crewe but later pardoned. In 1992, he directed Dark Horse and as an actor returned to the voyeuristic preoccupations of his Blowup character with a plum part as the Big Brother-esque villain in the season three opener for Tales From the Crypt.

In later years, he had roles in the blockbuster movie Gladiator (2000), playing Cassius, with Russell Crowe, and Last Orders (2001). One of his final film appearances was a cameo role in the cult hit, Equilibrium (2002), shortly before his death.