 The son of Joseph Abram Cassidy and Ethel McDonald, Cassidy was educated at Belfast High School and Queen's University, Belfast, where he received a Bachelor of Science degree in politics and economics in 1965.He was further educated in University College, London, where he graduated with a Master of Philosophy degree in 1967.He trained for ordained ministry with the Church of England at Oak Hill Theological College, London and was ordained in 1972.