Trewin was born in Plymouth, although both his parents were Cornish. He was educated at Plymouth College and in 1926 joined the Western Independent as a cub reporter. He moved to London in 1932 and joined the Morning Post, transferring to The Observer in 1937. He served as drama critic on the paper for over sixty years.
He was appointed Officer of the Order of the British Empire (OBE) in 1981.
He married Wendy Monk (1915–2000) in 1938 and they had two sons, Mark Antony Trewin and Ion Trewin (Literary Editor of The Times, Editorial Director of Hodder and Stoughton, Editor-in-Chief and Managing Director of Weidenfeld and Nicolson, 1997–2007, and Administrator of the Man Booker Prize from 2006).