Hamilton-Temple-Blackwood joined the 9th Lancers as a second lieutenant on 11 August 1897. He was promoted to lieutenant on 9 October 1899, and served with his regiment during the Second Boer War from 1899 to 1901, where he was present at the engagements at Belmont, Enslin, Modder River, Magersfonstein, the relief of Kimberley, and the advance to Bloemfontein and Pretoria. He was also present at the subsequent fighting in the Transvaal, Orange River Colony and Cape Colony, where he was badly wounded on Christmas Eve 1900. Twice mentioned in despatches (including 31 March 1900), he was awarded the Distinguished Service Order (DSO) in November 1900 for his service during the war.