Watkins was 66 when he discovered ley lines. This idea is that ancient manmade features of the landscape are placed in precise alignments which were "Old Straight Tracks". On June 30, 1921, Watkins visited Blackwardine in Herefordshire when his conception of a system of straight lines crossing the landscape since neolithic times came to him in a visionary flash. He presented his ideas to a meeting of the Woolhope Club of Hereford in September 1921. Henceforth he spent a major part of his life developing his theory. He published several books on ley lines and participated in the Old Straight Track Club from 1927 to 1930 (the papers from this organisation are also in the Hereford City Museum).
Watkins ideas were not accepted by contemporary archaeologists. One reason for this was that the prevailing opionion was that the ancient Britons were too primitive to have devised such a suggestion. This view has largely changed nowadays. Others claim that there is no reason to ascribe intentionality to the lay out of ancient monuments as such alignments would be produced by a random distribution of points. Watkins was sensitive to such arguments and argued for caution. He also drew up a list according to which landscape features could be given values between 1/4 and 1 point, five points or more being required as evidence of a ley line.
Watkins was a member of the Society for the Protection of Ancient Buildings and a fellow of the Royal Photographic Society and was involved in the preservation of Pembridge Market Hall.
Watkins work was revived and popularised from the 1960s following John Michell's publication of The View over Atlantis 1969. There is a journal, The Ley Hunter which discusses leys and other Earth Mysteries.