Keynes was born in London. He was the son of Zelfa Hourani and author Randal Keynes. He has an older sister, Soumaya Keynes (born 1989), who also starred in many productions of the BBC Radio 4. His grandparents by his mother’s side were Furugh Afnan and Cecil Fadlo Hourani. Cecil Fadlo Hourani used to be an author and also advisor to Habib Bourguiba, the late Tunisian president. Cecil Hourani was also the brother of Albert Hourani, a historian who studied about the Middle East. The Hourani family were immigrants to England.