By 1911, Mitchell was based at Bulford Barracks with the Royal Field Artillery. On 4 August 1914, the day Britain entered the First World War, Mitchell re-enlisted in the Royal Field Artillery in Scarborough as a gunner; he took part in the Battle of Mons in September and was wounded the following month. On 6 January 1916, having risen to the rank of sergeant, Mitchell was severely wounded during combat on the Western Front and was evacuated to No. 33 Casualty Clearing Station, Béthune, where he died of his wounds. He is buried at Béthune Town Cemetery Extension.