Stephen Roskill was born in London on 1 August 1903, the son of John Henry Roskill and Sybil Mary Wentworth Dilke. He was educated at Mr Egerton's School, 13 Somerset Street, London; Horris Hill School, Newbury, 1911-16; and the Royal Naval Colleges at Osborne and Dartmouth, 1917-20. He married Elizabeth Van den Bergh in 1930, with whom he had four sons and three daughters.

He served as a Midshipman in the cruiser HMS Durban on the China Station, 1921-4, and also acted as a research assistant to Lieutenant-Commander Stephen King-Hall, who was writing a book on East Asia and Western civilization. He was promoted Sub-Lieutenant and Lieutenant and served in the sloop HMS Wistaria on the North American and West Indies Station, 1925-6, and HMS Ramillies on the Home and Mediterranean Stations, 1927. He also spent some time as a German Language Officer at Freiburg-im-Breisgau, 1926-7. He was a student on the long gunnery course at the Royal Naval College, Greenwich, and HMS Excellent, 1927-9, and then joined the instructional staff at HMS Excellent, 1929-30. He served as a gunnery officer in the battleship HMS Royal Sovereign, 1930-2, the aircraft carrier HMS Eagle, 1933-5, and the battleship HMS Warspite, 1936-9, and returned as a gunnery instructor to HMS Excellent, 1935-6. He was promoted Commander, 1938; joined the Admiralty staff, 1939-41; and was then posted to the cruiser HMNZS Leander in the Pacific, 1941-3, which he later commanded as Captain, 1944. He was chief staff officer for administration and weapons in the British Admiralty Delegation in Washington, 1944-5; senior British observer at the Bikini Atoll atomic bomb tests, 1946; and Deputy Director of Naval Intelligence at the Admiralty, 1947-8. He retired from the Navy in 1948, due to increasing deafness caused by exposure to gun detonations.

Following his retirement, he pursued a career as a historian. He was official naval historian of the Second World War in the Cabinet Office Historical Section, 1949-60, and a Senior Research Fellow, 1961-70, and Pensioner Fellow, 1970-82, at Churchill College, Cambridge, where he was closely involved in the early development of Churchill Archives Centre.

He died in Cambridge on 4 November 1982.

He was awarded the DSC, 1944, and CBE, 1971.

His publications include: "Claude Francis Webster" (1945); "The War at Sea 1939-1945", three volumes (1954, 1957, 1960-1); "Escort, the Battle of the Atlantic" (1955); "HMS Warspite" (1959); "The Secret Capture" (1959); "The Navy at War" (1960); "The Strategy of Sea Power" (1962); "A Merchant Fleet in War" (1962); "The Art of Leadership" (1964); "Naval Policy Between the Wars", two volumes (1968, 1976); editor, "Documents Relating to the Naval Air Service" (1969); "Hankey, Man of Secrets", three volumes (1970, 1972, 1974); "Churchill and the Admirals" (1977); and "Admiral of the Fleet Earl Beatty, the Last Naval Hero" (1980).