He worked as an obstetrician, and gaining a great reputation as a physician, was elected Fellow of the Royal Society of London in 1706, FCP in 1721.
He was asked to investigate the case of Mary Toft, an English woman from Godalming, Surrey, who in 1726 became the subject of considerable controversy when she tricked doctors into believing that she had given birth to rabbits.
Douglas produced a series of manuscript English, French, Latin and Greek grammars, and an ample index to the works of Horace.
A Treatise on English Pronunciation by James Douglas (1914) was edited by Anna Paues.
As a result of Douglas's investigations of female pelvic anatomy, several anatomical terms bear his name: