Educated at Elizabeth College, Guernsey and the Royal Air Force College, Cranwell, Le Cheminant was commissioned into the Royal Air Force as a pilot officer (on probation) on 23 December 1939, shortly after the outbreak of war. He was confirmed in his rank and promoted to flying officer (war-substantive) on 23 December 1940. He was promoted to flight lieutenant (war-substantive) on 23 December 1941 and to the substantive rank of flight lieutenant on 23 June 1943.
An acting squadron leader by 1943, he was promoted to squadron leader (war-substantive) on 4 August 1943.
He was promoted to the permanent rank of squadron leader on 1 August 1947. He also served in the Korean War and was awarded a Bar to his Distinguished Flying Cross in 1951.
He was promoted to wing commander on 1 July 1951 and to group captain on 1 July 1958. Promoted to air commodore on 1 January 1964, Le Cheminant was appointed Senior Air Staff Officer, Far East Air Force on 16 May 1966 with the acting rank of air vice marshal.
He was promoted to the permanent rank of air vice marshal on 1 January 1967, and became Commandant of the Joint Warfare Establishment at Old Salisbury on 20 November and Assistant Chief of the Air Staff (Policy) on 1 May 1971.
Promoted air marshal on 4 July 1972, he then became Vice-Chief of the Defence Staff in 1974 He was promoted to air chief marshal on 2 February 1976 and appointed Deputy Commander-in-Chief of Allied Forces Central Europe on 5 February. He relinquished the command on 1 June 1979 and retired from the Royal Air Force on 27 August. He served as Lieutenant-Governor of Guernsey from 1980 to 1985.
He remains a keen Bisley marksman.