At the outbreak of the First World War Harrison was posted first to Harwich, manning anti-aircraft guns, and then to France, where he was involved in intense fighting around Hooge. He was promoted to Lieutenant in June 1915, and awarded the Military Cross later that year "for gallantry and initiative at Givenchy, on 10 March 1915, when he rushed up his gun under heavy shell fire and close-range rifle fire, and destroyed a hostile machine gun". He went on to fight at the Battle of Loos. He was promoted to temporary captain in November 1916, a promotion which was made permanent a year later. In 1918 he was promoted to brevet major, making him the youngest officer in the army to hold the rank at the time, and given command of an infantry school before appointment as a general staff officer first grade. By the end of the First World War, Harrison was attached to the 58th Division and had been mentioned in dispatches four times.
Between the wars
Following the end of the war, Harrison resumed his sports careers. He was selected for the "Mother Country" rugby team in 1919, and played for the army against Oxford University in 1920. Harrison's rugby career ended prematurely due to injury, after which he refocused on hurdling; he was selected for the 1920 Summer Olympics but was prohibited from competing due to heart problems. He was appointed master of the Royal Artillery Harriers and then selected for staff college. He went on to compete at the 1924 Summer Olympics in Paris, and reached a semi-final in the 110 metres hurdles. After the Olympics, Harrison attended the Staff College, Camberley from 1925 to 1926 and was appointed adjutant to Lahore District, India, from 1928 to 1932. There he became master of the Lahore Hunt and took up big game shooting and racquets, in which he won the doubles event at the All India Championships. He returned to England in 1932, when he was promoted to permanent major. In 1934, Harrison was appointed a battery commander at Catterick Garrison in Yorkshire. Here he began painting, and eventually had three of his works hung at the Salon in Paris. Later that year, he was promoted to brevetlieutenant colonel and given command of the Oxford University Officer Training Corps and lectured at the university on military history until he relinquished the post in 1938; he also became master of the South Oxfordshire Hunt. In 1939, Harrison briefly served in Mandatory Palestine, commanding an infantry battalion at the end of the Arab Revolt under Major-GeneralBernard Montgomery, General Officer Commanding8th Infantry Division and a former Staff College instructor, who remarked that, "having loosed 'Dreadnought Harrison' on the task of killing rebels he needed no urging in this respect". On 22 July 1939 Harrison was promoted to lieutenant colonel.
In retirement, Harrison lived in Cornwall. He became master of the North Cornwall Hounds and was appointed Deputy Lieutenant of Cornwall in 1955 and then High Sheriff of Cornwall in 1958. He also served as a justice of the peace, and chairman of St Lawrence's Hospital in Bodmin. In spare time, he grew rhododendrons and bred Labrador Retrievers, and fished for trout. He also published three books, including an autobiography, Gunners, Game & Gardens. His wife, Roza, died in 1967. Harrison himself died in Amesbury, Wiltshire, on 20 December 1987 at the age of 94.
