Geoffrey Keynes was the son of John Neville Keynes, an economics lecturer at the University of Cambridge and Florence Ada Brown, a successful author and a social reformer. His older brother was the economist John Maynard Keynes.
He graduated from Pembroke College, Cambridge, and then qualified as a surgeon with the Royal College of Surgeons in London. He served as a Lieutenant in the Royal Army Medical Corps during World War One and then worked as a consultant surgeon, becoming an expert in blood transfusions.
During World War II, Keynes was a consulting surgeon to the Royal Air Force. In 1944 he was promoted to the rank of acting air vice-marshal.
He also produced biographies of English writers such as Sir Thomas Browne, John Evelyn, Siegfried Sassoon, John Donne and Jane Austin.