Douglas served in the Second Boer War from 1899 until early 1901. He was initially Assistant Adjutant-General on the Headquarters staff in South Africa from 9 October 1899, then served as ADC and Chief of Staff to Lord Methuen, the General Officer Commanding 1st Division. It was in this role he took part in the Battle of Modder River (November 1899) and the Battle of Magersfontein (10-11 December 1899), in which the defending Boer force defeated the advancing British forces amongst heavy casualties for the latter. Douglas was mentioned in the despatch from Lord Methuen describing the battle. He was promoted to the local rank of major general and made Commander of the 9th Infantry Brigade on 10 February 1900, shortly before the Relief of Kimberley. Following the Siege of Mafeking, Douglas was appointed Commander of Mafeking and the adjacent district on 28 August 1900. The rank of major-general was confirmed for "distinguished service in the field" in early 1901 (but dated to the initial appointment on 10 February 1900).