Knott was born in Salford. His first acting was done in British television and radio programmes. He was trained in the Oldham Theatre Workshop. Knott's first major film was Warner Brothers' The Secret Garden in 1993, in which he played Dickon Sowerby. He went on to play Joe Green in Black Beauty. In the late 1990s, he played Liam Shepherd in Coronation Street.
In 2004, he returned to theatre to act in Alan Bennett's The History Boys. The National Theatre production later toured the world, affording Knott his Broadway debut. He reprised his role on BBC radio, and in Bennett's film adaptation which was directed by the National's Nicholas Hytner. In 2011, he appeared in "The National Anthem", an episode of the anthology series Black Mirror. Knott most recently appeared in the TV series Drop Dead Gorgeous, Gavin & Stacey and the BBC 3 drama series Spooks: Code 9 as Rob. Most recently he appeared in 1 episode of the LGBT miniseries Banana. He has also guest starred on the BBC show Father Brown.
