At age 12 he gave his debut in the British television series The Gentle Touch in 1980. In 1982 he appeared as Stanley Moon, the son of Harvey Moon, in the series Shine on Harvey Moon and as Falstaff's page Robin in The Merry Wives of Windsor. In 1983 he was seen in the series Dramarama.

In 1987 he had a short appearance in the British movie Wish You Were Here by David Leland and he became further notable in the television drama Two of Us about a gay relationship between two schoolboys. The following year he starred alongside Coronation Street star Chris Gascoyne in Central Television's series for schools, Starting Out.

In 1989 Whitlock played his most notable role, that of Sydney Chaplin, the elder half-brother of Charlie Chaplin, in the television movie Young Charlie Chaplin by Bazz Taylor. Further well-known series where he had roles were Split Ends, EastEnders, Harry Enfield's Television Programme, Lovejoy, Soldier Soldier, Boon, Grange Hill, A Touch of Frost, The Detectives, Silent Witness, McCallum, Casualty, London's Burning, and The Bill. In 2007 he played a policeman in Sweeney Todd: The Demon Barber of Fleet Street which starred Johnny Depp and Helena Bonham Carter. In 1992, Whitlock also starred in the W.I.Z. directed short film for Flowered Up's song "Weekender", playing the role of Little Joe.