English biographer, literary historian, editor, essayist, and critic, a wide-ranging man of letters who was an authority on Lord Byron.

Quennell was educated at Balliol College, Oxford. After practicing journalism in London, he taught at the Tokyo University of Science and Literature in Japan in 1930 but resigned after a year and returned to writing in London. He served as editor of the literary and artistic periodical The Cornhill Magazine (1944–51) and of the monthly journal History To-day from 1951. He was knighted in 1992.

Quennell's first volume of poems was published when he was 17, but he soon dropped poetry and fiction to become a literary reviewer and an author of biographies and histories. Among his most notable works are three biographical studies of Lord Byron: Byron (1934), Byron: The Years of Fame (1935), and Byron in Italy (1941). Quennell ranged widely through the history of English literature, writing works on John Ruskin, Alexander Pope, Queen Caroline (the consort of George II), Samuel Johnson, William Shakespeare, William Hogarth, and Vladimir Nabokov. His autobiography consists of Marble Foot (1976) and The Wanton Chase (1980).