Born in Gore in 1891, Hargest was a farmer when he volunteered for the New Zealand Expeditionary Force following the outbreak of the First World War in August 1914. Commissioned as an officer, he served in the Gallipoli Campaign in 1915 and was seriously wounded. Following his recovery from his wounds, he returned to active duty on the Western Front. He commanded an infantry battalion during the later stages of the war and received several awards for his leadership. After the war, he returned to New Zealand to resume farming. In Hargest entered the Parliament of New Zealand as the member for Invercargill. Initially an independent, he was one of the strongest supporters of the National Party that was formed in 1936, and held an executive role in the party hierarchy. From 1938, he represented the Awarua electorate and had been considered for the party leadership, but he was no longer available once he volunteered for active service.