Moores commissioned into the Royal Army Medical Corps as surgeon-captain on 1 February 1890, and served in the Chitral Relief Expedition in 1895, in medical charge of the 1st Battalion Gordon Highlanders and No. 8 Mountain Battery Royal Artillery. He served in the Second Boer War in South Africa (1899–1902), attached to the Scots Guards taking part in the Kimberley relief force, and was present at the battles of Belmont, Enslin and Modder River (November 1899), where he was wounded. For his service, he was promoted to surgeon-major on 29 November 1900, and twice mentioned in dispatches. Following the end of the war, Moores left Cape Town for England on the SS Simla in July 1902.
He served in France throughout the First World War running various hospitals before being appointed Director of Medical Services for the Second Army in April 1918, and later the same post for the Army of Occupation in Germany. He had responsibility for medical services at the British Empire Exhibition at Wembley in 1924. Guise-Moores was Colonel Commandant of the Royal Army Medical Corps between 1927 and 1933.
Guise-Moores was made a Knight of Grace of the Order of Saint John and a Knight Commander of the Order of the Bath in 1925. He was made Knight Commander of the Royal Victorian Order in the 1931 Birthday Honours.
- Hart′s Army list, 1903
- "The Army in South Africa - Return of Troops". The Times (36844). London. 12 August 1902. p. 10.
- "No. 33059". The London Gazette. 23 June 1925. p. 4913.
- "No. 33053". The London Gazette (Supplement). 3 June 1925. p. 3769.
- "No. 33722". The London Gazette (Supplement). 3 June 1931. p. 3628.