He was educated at the Royal Grammar School, Newcastle, and went on to take a French and Philosophy degree at New College, Oxford.

Alongside his acting career, Cullen has also worked as a director and as a teacher at Guildhall School of Music and Drama, the Royal Academy of Dramatic Art (RADA) in London, the American Conservatory Theater School (San Francisco) and the British American Drama Academy (London).

He has recently appeared in Happy Now?, a new play by Lucinda Coxon, and in Love the Sinner at the National Theatre (London).

His TV credits include: Outnumbered, Ghostboat, Midsomer Murders and Dalziel and Pascoe.

His film appearances include: Poppy Shakespeare, Fred Claus,[19] Finding Neverland[20] and Velvet Goldmine.