With the start of the Second Boer War, Robertson was appointed as Deputy Assistant Adjutant-General to Frederick Roberts, 1st Earl Roberts, the British Commander-in-Chief South Africa, on 15 January 1900. He was present at the Battle of Paardeberg (17–26 February 1900), the Battle of Poplar Grove (7 March 1900) and other battles in March and May. Robertson was promoted to major on 10 March 1900 and was mentioned in despatches on 2 April 1901.