Dennis Kelly grew up in Barnet, North London, in an Irish family and was brought up a Catholic. Kelly, one of five children, left school at 16 to work in Sainsbury's.
While working in supermarkets, he discovered theatre when he joined a local youth group, the Barnet Drama Centre. He took a degree in Drama and Theatre Arts when he was 30 and received a first from Goldsmiths College, London.
Kelly wrote his first play Debris when he was 30; he says he wrote it imagining he'd give himself a part. Staged at Theatre 503 in 2003, it transferred the next year to Battersea Arts Centre. It was well received and he went on to write the controversially titled Osama the Hero which was produced by Hampstead Theatre, beginning a long-running relationship with the theatre that he would return to often.