Peter Gretton was born 27 August 1912, the son of Major G. F. Gretton. He was educated at Roper’s Preparatory School and then the Royal Naval College, Dartmouth. In 1943 he married D. N. G. Du Vivier [Judy Du Vivier]; they had three sons and one daughter.

Joining the Navy as a cadet at Dartmouth [Devon] in 1926, Gretton showed early promise by winning a prize for Five First Class Certificates in his Sub-Lieutenant's courses. In the 1930's he served on a number of ships: the Royal Yacht Victoria and Albert (1934); the aircraft carrier HMS Courageous, then the cruiser Durban during the Abyssinian crisis in 1935, and the Spanish Civil War. In 1936, still with HMS Durban, Gretton was awarded the DSC for his actions during the Arab Rebellion in Palestine, where he helped the Army protect truck convoys and took a force ashore to help the police control Haifa. Gretton was then appointed to HMS Impregnable (the training establishment at Devonport [Devon]), before spending two years as a house officer at Dartmouth. In 1939 Gretton spent a week on an anti-submarine training course, which was to have a significant impact on his wartime service. When war broke out, he was appointed first lieutenant of the destroyer HMS Vega, escorting east coast convoys, then was transferred to the destroyer HMS Cossack for the Norwegian campaign of 1940. For Cossack's part in the second battle of Narvik, Gretton was mentioned in dispatches. He returned to convoy duty with his first command, the destroyer HMS Sabre, and in 1942 was promoted to lieutenant-commander of the destroyer HMS Wolverine: while escorting a convoy to Malta in August 1942, Wolverine rammed and sank an Italian U-boat, for which Gretton was awarded the DSO. At the end of that year, he was appointed to lead B7 Escort Group and in April 1943 the group fought a critical nine-day battle with up to 40 U-boats, and although Gretton's own ship had to withdraw, the remaining ships and their reinforcements sank six U-boats, with two more being lost in collisions. From October 1943 the group supported five more convoys across the Atlantic, sinking three U-boats without losing a single member of the convoys. From 1944 to 1946 Gretton left escort duty to work in the Admiralty plans division, being promoted to Captain in 1948. In the early 1950's he served first as Naval Assistant to the 1st Sea Lord, then commander of the cruiser HMS Gambia, before going to Washington [United States] as a naval chief of staff on the Joint Services Mission. From 1956-57 Gretton was Commodore in command of the naval task group for the atomic bomb tests at Christmas Island, being promoted to Rear-Admiral in 1958. Gretton was Senior Naval Member of the Directing Staff of the Imperial Defence College (1958-60) before serving as Flag Officer in command of sea training (1960-61), then a Vice-Admiral, a Lord Commissioner of the Admiralty, Deputy Chief of Naval Staff and 5th Sea Lord until his retirement in 1963. After retirement, Gretton took on the post of Domestic Bursar at University College, Oxford (1965-71) and then Senior Research Fellow at the college (1971-79). He was also Vice-President of the Royal Humane Society. His honours include: KCB 1963 (CB 1960); DSO 1942; OBE 1941; DSC 1936; MA.

Gretton's publications include: Convoy Escort Commander (1964); Maritime Strategy: A Study of British Defence Problems (1965); Former Naval Person: Churchill and the Navy (1968); Crisis Convoy (1974).

He died on 11 November 1992.