Carter-Campbell entered the Royal Military College, Sandhurst and was commissioned on 23 October 1889 as a second lieutenant into the Cameronians (Scottish Rifles). In the next decade he was promoted to lieutenant on 29 April 1892, and to captain on 2 November 1897. He served in the Second Boer War (1899–1902) as adjutant of the 2nd Battalion, which took part in the Ladysmith Relief Force, where he was present at the battles of Colenso (December 1899), Spion Kop (January 1900), Vaal Krantz and the Tugela Heights (February 1900). They served in Natal from March to June 1900, then in Transvaal east of Pretoria from July to November 1900. For his services he was promoted brevet major on 29 November 1900, and twice mentioned in despatches. Following the end of hostilities in South Africa, he returned to the United Kingdom in August 1902.