Campbell was commissioned a second lieutenant in the Lancashire Fusiliers on 23 November 1898, followed by promotion to lieutenant on 27 September 1899. He served with the 2nd Battalion of his regiment in the Second Boer War 1899–1901, including as part of the Ladysmith Relief Force, and was slightly wounded at the engagement at Venters Spruit (20 January 1900), for which he was created a Companion of the Distinguished Service Order (DSO). He was promoted captain, 5 October 1901.