Born in Wexford to a Protestant family, Brennan was educated at an English public school and Oxford where he read law. He was not an assiduous student, associating with the 'horsy' set in richer colleges and developing his life-long interest in the Turf. Soon after qualification as a solicitor the Second World war started and he was commissioned into the Royal Artillery. In 1942 his parents died in a house fire and Brennan was released from the army to take over his father's legal practice in Ireland. He took up writing, under the pen name of John Welcome, to relieve the tedium of the country solicitor's life.