Foot joined the British Army on the outbreak of the Second World War and was commissioned into a Royal Engineers searchlight battalion.

By 1942, he was serving at Combined Operations Headquarters, but wanting to see action he joined the SAS as an intelligence officer and was parachuted into France after D-Day.

For his service with the French Resistance he was twice mentioned in despatches and awarded the Croix de Guerre.

This led him to become the official historian of SOE, with privileged access to its records, allowing him to write some of the first, and still definitive, accounts of its wartime work, especially in France.

He was at one time married to the British philosopher Philippa Foot (née Bosanquet), the granddaughter of U.S. President Grover Cleveland.