[2] Nicholls appeared in several teenage stage roles, notably as Aladdin in Aladdin - Genie of the Ring at the Central Theatre in Chatham (1996–1997), and he made his London stage debut as Billy Fisher in the 1998 production of Billy Liar, at the King's Head Theatre in Islington.

[3] Nicholls' 2008 career began as the character Robert Fielding in Harley Street alongside Suranne Jones.

[6] Nicholls also appeared as Simon Marshall in the BBC hospital drama serial Holby City between May and July 2012, and played Cal Beckett in Channel 4's crime thriller, The Fear.

[7] He played the role of DS Sam Casey in ITV's Law & Order: UK until the end of series 7 in 2013.

In January 2019, Nicholls took over the role of Raymond from Mathew Horne in the British touring production of Rain Man.