After resigning from the university museum in 1957, Lethbridge moved with his wife to Branscombe, Devon. There he devoted himself to researching paranormal phenomena, publishing a string of books on the subject aimed at a popular rather than academic audience. Most of this involved his research into the use of pendulums for dowsing, although in other publications he championed the witch-cult hypothesis of Margaret Murray, articulated the Stone Tape theory as an explanation for ghost sightings, and argued that extraterrestrial species were involved in shaping human evolution; in this he came to embrace and perpetuate the esoteric ideas of the Earth mysteries movement. Although his work in parapsychology was derided and ignored as pseudo-scientific by the academic establishment, he attracted a cult following, and his work was posthumously championed by esotericists like Colin Wilson and Julian Cope. In 2011 he was made the subject of a biography by Terry Welbourn.