Keynes was born in London, the second son of Geoffrey Keynes and his wife Margaret, the daughter of George Howard Darwin who in turn was the son of Charles Darwin, making him the great-grandson of Charles Darwin (see Darwin — Wedgwood family). He was also the nephew of the renowned economist, John Maynard Keynes. His older brother Richard was a physiologist, and younger brothers Milo and Stephen both writers.
Keynes moved to the United States in 1939. Shortly after the Second World War he started his life as an explorer of Africa and sub-equatorial islands of the Atlantic and Indian Oceans. He made several films, and wrote several articles for the National Geographic Magazine. He collected books principally on the great explorers of the 19th century, but also travel, natural history and modern literature.
A biography of him was published in 2004 by his nephew, the historian Simon Keynes.