Warner served in the Home Guard during the Second World War and also worked as a Latin teacher at a Grammar School in Morden as there was a shortage of teachers. From 1945 to 1947 he was in Athens as Director of the British Institute. At that time he became involved in numerous translations of classical Greek and Latin authors. His translation of Thucydides' "History of the Peloponnesian War" for Penguin Classics sold over a million copies. He also translated "Poems of George Seferis" (1960).