Cator was commissioned a second lieutenant in the Scots Guards on 9 June 1897, and was promoted to lieutenant on 17 May 1899. He served with the 1st battalion of his regiment in South Africa during the Second Boer War, and was present at the battles of Belmont, Modder River, and Magersfontein in 1899. The following year, he took part in the march to Bloemfontein and Pretoria, the battles of Diamond Hill and Belfast , and advance to Komatipoort. Following the end of the war in late May 1902, Cator returned home with his regiment in the "SS Tagus", which arrived in Southampton in July.