He was born Reginald Ernest Warner in Birmingham, England and brought up mainly in Gloucestershire, where his father was a clergyman. He was educated at St. George’s School in Harpenden, and at Wadham College, Oxford, where he associated with W. H. Auden and Cecil Day Lewis, and published in Oxford Poetry. After graduating in 1928, he spent time teaching, some of it in Egypt. His first collection, Poems, appeared in 1937. He was a contributor to Left Review, and his first novel, The Wild Goose Chase, is in part a dystopian fantasy of a tyrannical government which is overthrown in a heroic revolution. His second novel, The Professor, published around the time of the Nazi Anschluss, is the story of a liberal academic whose compromises with a repressive government lead eventually to his arrest, imprisonment and execution "while attempting to escape"; contemporary reviewers saw parallels with the Austrian leaders Engelbert Dollfuss and Kurt Schuschnigg. After Why Was I Killed? (1943), Warner abandoned contemporary allegory in favour of historical novels about Ancient Greece and Rome, including Imperial Caesar for which he was awarded the 1960 James Tait Black Memorial Prize for fiction.
From 1945 to 1947 he was in Athens as Director of the British Institute. At that time he was involved in numerous translations of classical Greek authors. He also translated George Seferis (Poems of George Seferis,1960).
In 1983 the BBC screened an adaptation of The Aerodrome. It was written by Robin Chapman and directed by Giles Foster. The cast included Peter Firth as Roy, the protagonist; Richard Briers as the Rector; and Jill Bennett as Eustasia.