Reynell attempted to enlist in the Australian army to fight in the Second Boer War but was prevented from doing so by his father. Nonetheless, in July 1908, he became a member of the pre-war militia as a second lieutenant in the 16th Light Horse Regiment (South Australian Mounted Rifles), reaching the rank of major in the 22nd Light Horse Regiment in November 1912. Shortly after the outbreak of the First World War, Reynell volunteered to serve in the Australian Imperial Force. He rose rapidly through the ranks and was appointed as a major in the 9th Light Horse Regiment. During the Gallipoli Campaign, he took command of the regiment after its commanding officer was killed at the Battle of the Nek. Promoted to lieutenant colonel, he was killed shortly after on 28 August at the Battle of Hill 60, aged 31. He was buried in the Hill 60 British Cemetery and posthumously mentioned in dispatches on 5 November 1915.