Duncan began his television career as a child actor, appearing in series such as a BBC dramatisation of Treasure Island and the ITV children's science fiction series, The Tomorrow People. Unusually, he did two stints with Blue Peter, being on the team from 1980 until 1984 and again in 1985-1986. During his stay with the programme, he became associated with the kind of daredevil stunts previously popularised by John Noakes. After 1986 Duncan returned to the stage, most notably in the musical Barnum.