Duncan's early career was as a stage actor, appearing as Jim Hawkins in Treasure Island followed by two years at Sir Laurence Olivier's National Theatre.

His notable television roles include work on The Tomorrow People, Space: 1999,[3] King Cinder,[4] Play for Today, Warship, Oranges & Lemons, and season 2 of Survivors.

Duncan starred in an episode of the children's television programme Storybook International as a young master in the tale of 'Cap O Rushes'.

Duncan played leading roles in the BBC's King Cinder as Kerry, and as Jimmy Carter in the 1977–1978 ITV children's television series The Flockton Flyer.

He joined Blue Peter in 1980, and shortly afterwards it was revealed in a tabloid newspaper that Duncan had appeared nude in the Michael Papas psychological thriller film The Lifetaker.

During his stay with the programme, he became associated with the kind of daredevil stunts previously popularised by John Noakes, and these continued in his own spin-off series, Duncan Dares.

One of his most remembered such stunts occurred in 1980 when the Blue Peter crew put him out on a rope swing to clean the face of Clock Tower (Big Ben) without a safety harness, thus creating much trouble for the BBC.

In the 1990s he appeared in numerous pantomimes, some of which he also produced through his company Here's One I Made Earlier, including Robin Hood at the Lewisham (now Broadway) Theatre in South East London.

[14] In 2007, he played Macduff in Macbeth at Regents Park Theatre, London,[15] and filmed a documentary about the Scouts climbing Everest.