Harcourt's military career began in 1914 when he was commissioned into the British Army. During the First World War he served on the Western Front, commanding a machine gun unit. He gained promotion quickly, advancing to the rank of major by the end of the war. He also received a number of notable decorations. Later, Harcourt served during the Russian Civil War before emigrating to Australia in 1929. Following this, he worked as a public servant in Tasmania before the outbreak of the Second World War when he joined the Australian Army and subsequently went on to serve in the Australian commandos, taking part in the fighting in New Guinea and Borneo.