Loch was seconded from his regiment to the staff on 9 October 1899, serving as divisional signalling officer, South African Field Force in the Second Boer War. He was promoted captain on 28 January 1900, this was subsequently backdated to 30 November 1899. He inherited the title Baron Loch on the death of his father in 1900. He was Mentioned in Despatches again in April 1901, and on 19 April it was announced he would receive a brevet promotion to major, dated 29 November 1900. He participated in the battles of Belmont, Enslin, Modder River and Magersfontein, was badly wounded and received the Queen's South Africa Medal with four clasps.