He is the son of actor and comedian Keith Allen and the brother of singer Lily Allen. His most notable role so far has been a well-received 2009 stint as the lead in the play Equus.
He stars in HBO's Game of Thrones as Theon Greyjoy. He was announced in the role on 20 August 2009. He was part of the initial starring cast and has remained a member appearing on all subsequent seasons since.
Allen explained that Theon hasn't seen his own father Balon Greyjoy in nine years since he was a small child, so logically he doesn't know him very well. Therefore, as part of method acting, Allen intentionally avoided actor Patrick Malahide, who plays Balon, at all times except for when they were actually shooting scenes in front of the cameras, to maintain a level of unfamiliarity and uncertainty between them.