Following the outbreak of the First World War in August 1914 Hargest volunteered to serve in the New Zealand Expeditionary Force (NZEF) and was commissioned as a second lieutenant in the Otago Mounted Rifles. He served in the Gallipoli Campaign and was severely wounded during the August Offensive. After several months of convalescence, he returned to active service in July 1916 with the New Zealand Division. Assigned to the 1st Battalion of the Otago Infantry Regiment, he commanded a company during the Battle of the Somme in September 1916. His actions in restoring order in his battalion, when he assumed command of four companies that had suffered heavy casualties following a failed attack on 27 September, saw him rewarded with the Military Cross. By the end of the year he had been promoted to major.