A prolific writer on Theosophical subjects and a dedicated worker for the Theosophical Society. He was born at Coggeshall, Essex, England. He joined the Society on April 17, 1907, was General Secretary of the English Section in 1924-8, and travelled widely as an international lecturer. In 1926 he founded a Theosophical community at Stamford House, Wimbledon, London, and presided over it until 1940. Gardner was one of a group that bought Tekels Park, now vested in the English section of the Society. His first wife was Clara Beard, who died in 1920; in 1922, he married Eliza Adelaide Draper.

Publications include The Fourth Creative Hierarchy (1913), Matter Is the Shadow of Spirit (1918), The Web of the Universe (1936), The Play of Consciousness (1939), This World and the Next (1941), The Mysteries (1945), Chains and Rounds (1948), Fairies: The Cottingley Photographs and Their Sequel (2nd ed. 1951), The Heavenly Man (1952),and A Mind to Embrace the Universe (1960).