Corlett was born in Darlington, County Durham. He was educated at Fettes College, Edinburgh, then trained as an actor at the Royal Academy of Dramatic Art. He worked as an actor while embarking on a literary career during the 1960s, and wrote plays and adult novels as well as the children's novels for which he is particularly remembered. Several of his works were adapted for the screen.
Later in life he came out as gay, and it was from his partner, Bryn Ellis, that he gained some of his inspiration for The Magician's House. Corlett died of cancer at Sarlat in France.