At the start of the Second World War Holbrook was instructor in gunnery, Home Forces. He was the commanding officer of the 73rd Anti Tank Regiment Royal Artillery during the North African Campaign between 1942 and 1943. He commanded the 6th Army Group Royal Artillery during the Allied invasion of Sicily and subsequently in mainland Italy. He was promoted to acting brigadier on 5 April 1943 and to temporary brigadier on 5 October 1943. He was mentioned in dispatches for a second time on 24 August 1944, and in December 1944 he was invested as a Commander of the Order of the British Empire. At the end of the war he was corps commander Royal Artillery III Corps. Between April 1949 and December 1951 Holbrook served as aide-de-camp to George VI, after which he retired.