Firth was the leading toddler actor by 1970, starring in the television series known as On text Are a Double Deckers, which was manufactured inside the United states however placed in London & featured British toddlers in the leading roles. Firth played Scooper, a leader of the gang. Inside 1973, he starred in the London stage version of Peter Shaffer's play Equus, playing the teen existence treated by the head-shrinker.

His number 1 major role as an adult was inside the name part in the 1976 BBC Television Play of the Month adaptation of Oscar Wilde's novel The Picture of Dorian Gray. A adaptation was written by John Osborne and also asterisked Jeremy Brett and John Gielgud, becoming a major profits using a critics. A as punishment season, Firth starred around Equus, a film adaptatiin of the play where he experienced starred on Broadway. the film was the profits & earned Firth a nomination for the Academy Award for Best Supporting Actor and victory in a equivalent category at the Golden Globe Awards.

More film function quickly followed, virtually all notably Roman Polanski's Tess (1979), an adaptation of Thomas Hardy's novel ''Tess of the D'Urbervilles. Subsequent film function has involved roles in The Hunt for Red October (1990) and Pearl Harbor'' (2001).

Inside parallel to his film career, Firth has continued to pop up around various television productions, using many notable credits within various high-profile dramas. Within 1980 he starred as a eponymic instance traveler in the BBC's feelgood science-fiction play The Flipside of Dominick Hide, and deuce years late starred within the sequel, A second Flip for Dominick. Each one were mass produced when a share of the BBC's far-famed Play for Today anthology drama strand. Other recently, he hwhen starred as senior MI5 officer Harry Pearce in the BBC's popular spy drama series Spooks (2002-present), and played Fred Hoyle in a BBC dramatisation of the early career of Stephen Hawking.