Born the son of Colonel Charles Seymour Corkran and educated at Eton College, Corkran was commissioned into the Grenadier Guards in March 1893. He was promoted to lieutenant on 12 May 1897, and served on the Nile Expedition in 1898, including at the Battle of Omdurman. He was appointed adjutant of the 2nd Battalion of his regiment on 22 October 1899, was promoted to captain on 30 November 1899, and left with his regiment for South Africa to serve in the Second Boer War in March 1900. Serving there throughout the war, he was wounded in the action at Biddulphsberg (May 1900), and was mentioned in despatches. For his service in the war, he received a brevet promotion as major in the South African Honours list published on 26 June 1902. Following the end of hostilities in June 1902, Corkran stayed in South Africa as Aide-de-camp to Sir Neville Lyttelton, who had been appointed Commander-in-Chief of South Africa that month. He resigned as adjutant of the 2nd battalion on 5 November 1902.