Born the son of a clergyman in George in South Africa, Warner was educated at Brighton College after his family returned to Britain in 1929. He decided to become an actor while watching his father in an amateur production of the play "Berkeley Square" on Worthing Pier. His first job in 1939 was at the Little Theatre in Bristol. After service in the Royal Navy during the Second World War on board "HMS Rattlesnake" (he rose to the rank of Lieutenant), which included working on the Russian convoys, he resumed his acting career.