Bennett was born in Garnant, Carmarthenshire, Wales, the son of Sarah Gwen (née Lewis) and Gorden Bennett. He was the brother of actor Alun Lewis, who is best known for playing Vic Windsor in Emmerdale. Bennett was brought up in London from an early age, and attended Sunnyhill School, Streatham, Henry Thornton Grammar School, Clapham (1955–62) and the Royal Academy of Dramatic Art.
Bennett debuted on stage in the role of Ophelia in a Queen's Theatre production of Hamlet in 1959.
On TV, Bennett played small parts in Doctor Who (1965) and The Sweeney (1976).
He played major parts in several Dennis Potter television plays, including Where the Buffalo Roam (1966), an episode of 1978's Pennies from Heaven, (Better Think Twice), in which he played Tom, a pimp, Karaoke (1996), and Cold Lazarus (also 1996). He took the lead role in Shelley (1979–84) and its sequel The Return of Shelley (1988–92).
He starred in The Virgin Soldiers (1969), the film adaptation of Loot (1970) by Joe Orton, and Percy (1971). He also starred opposite Hayley Mills in several films, including The Family Way (1966), Twisted Nerve (1968) and Endless Night (1972). Contemporary critic Roger Ebert called him “one of England’s best young actors.”
He appeared in the 1986 music video of "Loving You's a Dirty Job But Somebody's Gotta Do It" by Bonnie Tyler and Todd Rundgren. He playbacked the Todd Rundgren vocals. The song can be found on Bonnie's album Secret Dreams and Forbidden Fire.
He was also the villainous Mr Croup in Neil Gaiman's serial Neverwhere (1996). Bennett also starred in the four-part BBC mini-series Malice Aforethought (1979) based on a novel written in the 1920s by Francis Iles. Bennett's character, Ricki Tarr, was pivotal in the BBC serial based on John le Carré's novel Tinker, Tailor, Soldier, Spy (1979). He also appeared in Lock, Stock... (2000) as Deep Throat.
In 2003 Bennett joined the cast of the long-running soap opera EastEnders as Jack Dalton, a gangland boss who was a former adversary of Den Watts (played by Leslie Grantham) and was believed to have ordered Watts's death in 1989. He first appeared in the series in May 2003, but was written out just two months later when his character was shot dead by Dennis Rickman (Nigel Harman), son of Den Watts. Just before his death Dalton revealed that Den had survived the shooting 14 years earlier, and later that year Den returned to the series.
From 1970 to 1988, Bennett was married to Cathy McGowan, who was the presenter of the TV rock show Ready Steady Go! in the mid-1960s. They had a daughter, Emma. From 1998 he was married to Sandra Layne Fulford. Bennett retired from acting in 2007, and later moved to Deal, Kent. On 3 August 2017, it was announced that he had died on 25 July at the age of 73.
