Stirling served in the Second Boer War in South Africa throughout the war from 1899 until 1902. He was promoted captain on 29 January 1900. He took part in operations in the Orange Free State March to May 1900, including the battles of Poplar Grove, Driefontein and Sanna's Post (March 1900), Houtnek, Vet River and Zand River (May 1900); then took part in operations in Transvaal May and June 1900, including actions near Johannesburg, Pretoria and the battle of Diamond Hill (June 1900). Appointed adjutant of the Burma Mounted Infantry on 15 June 1900, he saw further action in the Orange River Colony and the Transvaal until October 1900, during which he was slightly wounded (26 September 1900).