Guy Arnold was one of five children by George Arnold and Rita Shaw; he also had two half siblings (one deceased). He was an older brother to Bruce Arnold, a novelist, non-fiction writer and journalist based in Dublin. Although born on Merseyside, he was brought up at the family home in Croydon and went to Kingham Hill School, a boarding school in Oxfordshire, thence to Chipping Norton Grammar School, before going on to St Peter's College, Oxford where he read History graduating in 1955.
