He is known for his roles as Max in Guilt, Duncan Hunter in Shetland, Bruno Jenkins in Casualty, Detective Finney in Psychoville, DCC Mike Dryden in Line of Duty, Colin Osborne in Unforgotten, Blackbeard in Assasin's Creed IV: Black Flag, Townsend in Battlefield 1 and Field in Summer of Rockets.
On television, Bonnar has appeared as Peter Mayhew in BBC One's New Blood and Chris in the Channel 4 comedy Catastrophe, a role which he reprised in the following series.
Other television credits include Vera, Grantchester, Case Histories, The Paradise, Doctor Who, Psychoville, Taggart, Phoneshop and Paradox.
Bonnar provided the voice and motion capture for Edward "Blackbeard" Thatch in the 2013 video game Assassin's Creed IV: Black Flag.
He also features in the Doctor Who series Doom Coalition from Big Finish Productions, where he portrays the Eleven, a villainous Time Lord who retains the personalities of his past incarnations in his mind.