James McMahon (April 22, 1856 – June 1, 1922) was an Irish mathematician whose career was spent at Cornell University in Ithaca, New York. He was a committed educator, and an early proponent of professionalization in the teaching of advanced mathematics in America. A professor and Chairman of the Mathematics Department in Cornell University's College of Arts & Sciences, McMahon was one of the earliest members of the American Mathematical Society (as the predecessor New York Mathematical Society) in 1891. For seven years he served as associate editor of the Annals of Mathematics. He was also the American Association for the Advancement of Science's Secretary (1897), Section A (Mathematics and Astronomy); General Secretary (1898), and Vice-President (1901). McMahon was also featured in the publication, American Men (and Women) of Science.