Owen-Allen was born in Hammersmith, London, the son of film producer Alison Owen and Welsh-born comedian Keith Allen. Keith Allen forms part of the pop band Fat Les, and Alison Owen has produced films such as Elizabeth, Sylvia, and 2005's Proof. His older sister, Lily Allen, is an English singer-songwriter. "Alfie", the eleventh song on her debut album Alright, Still is about him.

Owen-Allen went to St Johns College in Southsea, Portsmouth; and Fine Arts College, Hampstead where he studied for his A levels.

He has been dating Jaime Winstone since 2007. This relationship reportedly ended in January 2010.

Allen performed in a one-off Channel 4 comedy production, You Are Here in 1998, co-written by Matt Lucas and David Walliams. His father Keith Allen, Sally Phillips and Jon Ronson were also involved. Owen-Allen appeared alongside his sister, Lily Allen, in the 1998 film, Elizabeth produced by their mother, Alison Owen. They are credited as Arundel's Son and a Lady-in-waiting respectively.

He has appeared in small roles in Elizabeth, produced by his mother, and Agent Cody Banks 2: Destination London directed by his uncle Kevin Allen, and in the film Atonement, an adaptation of the Ian McEwan novel. He has also appeared in BBC1's historical hospital drama, Casualty 1907, as Nobby Clark.

Starting in Chichester on 31 January 2008, he took over Daniel Radcliffe's role in a revival of Equus on a nationwide tour.

In April 2009 he co-starred with Winstone in the music video for Dust Devil by Madness. He also had a role in BBC2 drama Freefall.

He appears in a number of forthcoming films: Powder, The Kid, Soulboy and the short Jenny..

It was announced on August 20, 2009 that Allen would star as Theon Greyjoy in the HBO series Game of Thrones, based on George R. R. Martin's A Song of Ice and Fire series of fantasy novels.