In August 1822, the first part of the Anatomical and Physiological Commentaries appeared, a work containing Mayo's assertion of his discovery of the real function of the nerves of the face, and his account of the experiments which proved it.

This was the starting point of a bitter and prolonged controversy with Sir Charles Bell, the discoverer of the distinction between sensory and motor nerves.

Dr. Whewell, in a letter to the London Medical Gazette dated 11 December 1837, describes the discovery as having been made by Bell, Mayo, and Majendie, the two latter physiologists having corrected and completed the researches of the former.

In 1843 gradually increasing rheumatic gout reduced him to a state of helplessness, and compelled his retirement from his duties as lecturer on surgery at the Middlesex Hospital, after six years' tenure of the post.

In the later years of his life he had thrown himself into the hands of the mesmerists, and his work on the Truths contained in Popular Superstitions is an ably written exposition of his views regarding the supposed cause of mesmeric and kindred phenomena.