 The son of Robert Hamilton-Udny, 11th Lord Belhaven and Stenton, by his marriage to Kathleen Gonville Bromhead, the young Hamilton was baptised Robert Alexander Benjamin.He was educated at Temple Grove School, Eastbourne, then at Eton College, before training for an army career at the Royal Military College, Sandhurst.He was an officer of the Royal Scots Fusiliers between 1924 and 1931, then was seconded to the Aden Protectorate Levies between 1931 and 1934, during which time he commanded the unit's Camel Troop.