Denys Rayner was born in Muswell Hill in a Georgian house, on the outskirts of London, to Francis (née Parker) and Arthur Rayner, a master electrical engineer, who became a commodity broker moving with his family to West Kirby on the Wirral Peninsula and sending his son to Repton School between 1921 and 1924. Flat feet kept Denys from entering the Royal Navy but not from pursuing a successful naval career. In October 1925, he joined HMS Eaglet, Mersey Division of the Royal Naval Volunteer Reserve (RNVR) as a part-time midshipman with time to pursue his interest in exploring the rocky coast of the Western Highlands in a small boat of his own design.