Tarn was cast as Pete in A Clockwork Orange and was the only actor in the gang who was a true teenager (16–17 years old) at the time of production, the others being in their mid-to-late 20s.

After guest appearances in a succession of TV series, he was cast in Where There's Brass for Yorkshire Television.

Unknown to him his then agent had negotiated him out of the series and his career as a film and TV actor was effectively over.

He made a few brief appearances over the next 20 years, including Crimewatch, The Knock, and the final one in 2000 when he played the part of Vic in Shooters for Coolbean Productions, directed by Colin Teague and written by and starred in by Louis Dempsey and Terence Howard with Emily Watson, Gerard Butler, amongst a host of other well-known British actors.

Amongst his theatre credits included spells with both the RSC and National Theatre Companies, with critically acclaimed parts as Rick in Sticks and Bones with Peter Weller, Rex in City Sugar by Stephen Poliakoff at the Comedy Theatre with Adam Faith, Jaques in Jaques and His Master by Milan Kundera, and Sam in Crossing Delancey by Susan Sandler.