Warner was born in 1903 and educated at Denstone College and Caius College, Cambridge. In 1926 he succeeded Frank Swinnerton as staff reader at the publishing house of Chatto and Windus. In addition to his work as staff reader he also wored on the company's advertising material. As a young man he made contributions to magazines such as The Spectator and Time and Tide, some of which were later reproduced in his 1947 book Captains and Kings. In 1939 he published an account of his visit to an "unworldly" relative in Canada, entitled Uncle Lawrence. During the Second World War he joined the Admiralty secretariat, initially serving in the Commission and Warrant (C.W.) branch before serving on the war artists advisory committee. He later served as secretary to the naval honours and awards committee.

After the war he became deputy director of publications of the British Council, where he remained until his retirement in 1963. he worked thereafter at Chatto and Windus for another year before concentrating on writing. By the time of his death he had more than twenty books in print. He married twice, first to Dorothea Blanchard who died in 1937, by whom he had one daughter. He was married secondly to Elizabeth Strahan, with whom he had one son and one daughter.

Warner died at his home, Old Manor Cottage, Haslemere, on 14 August 1976. A memorial service was held on 21 October (Trafalgar Day) at St. Lawrence Jewry-next-Guildhall church. As well as family members, the naval historians Professor Christopher Lloyd and Captain Stephen Roskill were in attendance, among others.