1. Allan Adair joined the British Army, receiving his commission as a probationary second lieutenant on 2 May 1916 in the 5th Battalion of the Grenadier Guards, the same regiment with which an ancestor of Adair's had been serving when he was killed at the battle of Waterloo.
Allan Adair took no part in the battle, due to an injury sustained in a bicycle accident in early July 1917.
Allan Adair returned to the battalion in January 1918, by which time he discovered that there only four of his fellow officers still with the battalion.
Allan Adair was awarded his first Military Cross on 2 December 1918.
Allan Adair led his company correctly into position and then made several personal reconnaissances under heavy machine-gun and rifle fire, and cleared up the situation.
Allan Adair captured the objectives without the assistance of tanks or artillery, and broke up a hostile counter-attack the following morning.
Allan Adair was promoted to major on 22 May 1932, and returned to the 3rd Battalion to serve as second-in-command until April 1940, seven months after the Second World War broke out.
Lance Corporal Harry Nicholls from Allan Adair's battalion was awarded one of the first Victoria Crosses of the war.
Allan Adair, awarded the Distinguished Service Order for his services in Belgium and France, was promoted to lieutenant colonel on 19 September 1940.
From 12 September 1942 until December 1945 Allan Adair was General Officer Commanding of the Guards Armoured Division, taking over from Major-General Oliver Leese, receiving promotion to colonel on 30 June 1943, while serving as an acting and then temporary major-general from 21 September 1942.
From December 1945, three months after the surrender of Japan, until November 1946 Allan Adair served as GOC of the 13th Infantry Division, during the Civil War, and receiving promotion to major-general on 25 July 1946, with seniority from 12 November 1944.
Allan Adair finally retired from active service on 11 March 1947, but remained in the Regular Army Reserve of Officers until reaching the mandatory retirement age on 3 November 1957, his 60th birthday.
Allan Adair was appointed Exon in the Yeomen of the Guard, the ceremonial bodyguards to the monarch, on 21 November 1947, receiving promotion to Ensign on 30 June 1950 and then to Lieutenant on 31 August 1951, before finally retiring on 14 November 1967.
Allan Adair served as a Governor of Harrow School from 1947 until 1952, was Colonel of the Grenadier Guards from 1961 to 1974, and a Deputy Grand Master of the United Grand Lodge of Freemasons from 1969 to 1976.
Allan Adair served as Deputy Lieutenant for County Antrim, and as a Justice of the Peace for the county of Suffolk.
On 28 April 1919 Allan Adair married Enid Violet Ida Ward.
Allan Adair succeeded his father as 6th baronet on 9 October 1949 inheriting the family home of Flixton Hall in Suffolk.