Lester was born in Oxford, Oxfordshire, the son of actress Rita Lester (née Keene) and actor and producer Michael Lester (originally Michael Boris Letzer). He attended the independent Halliford School in Shepperton, Middlesex and the independent Corona Academy stage school in Hammersmith, London, and had supporting roles in several British television series, including The Human Jungle and Danger Man.

At the age of nine, Lester was cast in Lionel Bart 's musical Oliver! (1968). He played the title character in the multiple Academy Award-winning musical adaptation of Charles Dickens' novel, which co-starred Jack Wild, Ron Moody and Shani Wallis It was directed by Sir Carol Reed.

These two child actors later reunited for Melody (1971), which depicted British schoolchildren in love. Tracy Hyde played the role of Melody in the film, which used early music from the Bee Gees and Crosby, Stills, Nash & Young.

In 1969, Lester received critical acclaim for his portrayal of a withdrawn only child in Run Wild, Run Free, starring opposite Sir John Mills. However, after starring in Eyewitness (1970), with Susan George and Night Hair Child (1971) with Britt Ekland, Whoever Slew Auntie Roo? (1971), with Shelley Winters; Melody and Black Beauty (1971), he found good roles harder to come by in England.

He extended his range in a series of films in Italy. These included Redneck (1972) with Telly Savalas and the excellent Western Scalawag (1973), with Kirk Douglas. The final film of his Italian career was in the costume drama La Prima volta sull'erba (Love Under the Elms) (1974), which was nominated for the Best Film prize at the Berlin Film Festival.

Lester finished his film career in England, with the lead role in Crossed Swords (UK title: The Prince and Pauper) (1977), which reunited him on screen with Oliver Reed, who had been Bill Sykes to his Oliver Twist.