 Hirst was commissioned as a second lieutenant in the 7th Battalion, Duke of Wellington's Regiment (Territorial Army) on 27 June 1934, and promoted to lieutenant three years later.A Captain on the outbreak of the Second World War, he was appointed adjutant on 1 October 1939.He transferred to the Royal Army Ordnance Corps (RAOC) as a Mechanical Engineering Officer on 14 November 1941, and to the Royal Electrical and Mechanical Engineers (REME) on its formation on 1 October 1942.