After graduating from Sandhurst, Kearsey was commissioned into the York and Lancaster Regiment as a second lieutenant on 7 May 1896. He was promoted to lieutenant on 20 June 1900. Kearsey fought in the Second Boer War, being present at the Relief of Ladysmith. During the operations in the Relief, Kearsay was severely wounded at Venters Spruit on 20 January 1900. The following month, having recovered from his injuries sufficiently enough, he took part in action at Spion Kop, Vaal Kranz and toward the end of February, action at Tucela Heights and Pieters Hill. Kearsey later took part in operations in Natal from March to June 1900, including at the Battle of Laing's Nek. He also took part in operations in Transvaal Colony from May to June 1901 and operations in Orange River Colony from December 1901 to 31 May 1902. Mentioned twice in dispatches during the course of the war by Sir Redvers Buller, he was also mentioned in the "London Gazette" in a list of soldiers and officers who had distinguished themselves in the conflict. Weeks later he was made a Companion of the Distinguished Service Order, with the insignia being presented by Edward VII on 14 March 1902. Following the end of the war in June 1902 he left Cape Town on board the SS "Rippingham Grange", which arrived in Southampton in October 1902.