Templeman was born on 3 March 1920, the son of Herbert William Templeman (a coal merchant) & his wife Lilian née Pheasant. He was educated at Southall Grammar School and St John's College, Cambridge, where he was a scholar and read History. His studies were interrupted by World War II. In 1941 he was commissioned into the 4 Gorkha Rifles, and saw action on the Northwest Frontier, at Arakan, Imphal, and Burma. For his wartime service, he was mentioned in dispatches, and was demobilised as an honorary Major, and then later appointed an MBE for his war service.