[5] Le Fanu served in the Second World War being posted to the cruiser HMS Aurora operating in the Home Fleet as gunnery officer in December 1939.

[5] He was given command of the aircraft carrier HMS Eagle in February 1957 and was promoted to rear admiral on 7 July 1958[12] on appointment as Director-General, Weapons at the Admiralty.

[15] Advanced to Knight Commander of the Order of the Bath in the 1963 Birthday Honours[16] and promoted to full admiral on 29 September 1965,[17] he became Commander-in-Chief, Middle East in December 1965.

[15] Advanced to Knight Grand Cross of the Order of the Bath in the 1968 New Year Honours,[18] Le Fanu became First Sea Lord and Chief of the Naval Staff in August 1968.

[15] He was nominated for the post of Chief of the Defence Staff but never held the office because he was suddenly discovered to be terminally ill with Chronic Lymphocytic Leukaemia (CLL).