Hoare was born on Saint Patrick's Day in Calcutta to Irish parents. His father was a river pilot. At the age of eight he was sent to school in England to Margate College and then commenced training in accountancy and, as he was not able to go to Sandhurst, he joined the Territorial Army. Hoare's childhood hero was Sir Francis Drake. Aged 20 he joined the London Irish Rifles at the outbreak of the Second World War, later he then joined the 2nd Reconnaissance Regiment of the Royal Armoured Corps as a 2nd lieutenant and fought in the Arakan Campaign in Burma and at the Battle of Kohima in India. He was promoted to the rank of major. In 1945, he married Elizabeth Stott in New Delhi, by whom he had three children. A short man, Hoare was described by those who knew him as "dapper" and "charming".