Beginning his career as a child actor, he has played various roles across theatre, stage and television, including as Sky Masterson in Michael Grandage's revival of Guys and Dolls and as Lord Farquaad in the original London production of Shrek the Musical, for which he received an Olivier Award.

[6] He had minor parts in episodes of Red Cap, Doctors and Coupling, but had to support himself by taking jobs at Pizza Hut and as a driver for supermarket chain Sainsbury's[7] In 2003, he was cast in the high-profile role of Dennis Rickman in the BBC soap opera EastEnders.

[8] Harman left EastEnders on 5 November 2005, and his final episode was aired on 30 December 2005, in which Dennis was stabbed to death in the street by one of Johnny Allen's (Billy Murray) henchmen,[9] later revealed to be Danny Moon.

After leaving EastEnders in December 2005, Harman appeared as Sky Masterson in Guys and Dolls at the Piccadilly Theatre,[10] in the West End, alongside Jenna Russell and Nigel Lindsay.

[10] In October 2006, Harman appeared in the Harold Pinter play The Caretaker at the Crucible Theatre in Sheffield,[10] starring alongside David Bradley and Con O'Neill.

[10] From September 2012 to January 2013, Harman he starred alongside Rob Brydon and Ashley Jensen in Alan Ayckbourn's A Chorus of Disapproval at the Harold Pinter Theatre in the West End.

[14] The same year, Harman was revealed as the director for the UK tour of Shrek The Musical,[10] which opened in July in Leeds and finished in February 2016 in Salford.

[10] From January to February 2017, Harman starred alongside Sarah Hadland, Jamie Glover, Olivia Poulet and Raymond Coulthard, in the French comedy What's in a Name?

[10] Harman directed the London premiere of the musical Big Fish, starring Kelsey Grammer, which played at The Other Palace in November and December 2017.

[10] Harman played the lead as Robert Langdon in the stage adaptation tour of Dan Brown's The Da Vinci Code, which opened in January 2022 in Bromley.

[10] In 2009 he appeared in the series Plus One,[10] and took over from Max Beesley as the new male lead in the BBC drama Hotel Babylon playing new owner Sam Franklin.

[10] The BBC announced in February 2023 that Harman had joined the cast of Casualty as Dr Max Cristie, the new clinical lead of the emergency department.