McMahon was the author of numerous liturgical, religious education and other non-fiction works, including Lectures on Education (1924), a series of small church service pamphlets (1949) and Pray the Mass : a Text-Book on the Mass (1974). He also wrote biographies of Catholic Church leaders and other works about the Catholic Church, including A Sketch of the Church in WA (1925), Ramblers from Clare and Other Sketches (1936), Bishop Salvado : Founder of New Norcia (1943), A Little Harvest (1944), One Hundred Years : Five Great Church Leaders (1946), The Durack Legacy and Other Ramblers from Clare (1980) and Success Stories (1981).
There is a detailed (and anonymous) biography of this author on the back cover of his religious education book To-Day - the Magic Word (1973), which states 'Monsignor J. T. McMahon was ordained a priest in All Hallows College, Dublin, in 1919. He graduated in Arts at University College, Dublin, in 1915 and was awarded second class Honours in 1917 at the Higher Diploma in Education and first class Honours in the Master's degree in Education in 1920. In 1926 he enrolled at the Catholic University of America, Washington D. C., in the post-graduate School of Education. In 1928 he received a Ph.D. for his thesis, later published as Some Methods of Teaching Religion. On his arrival in Perth, Western Australia in 1921 he became Diocesan Inspector of Schools and chairman of the Diocesan Council of Education. The Bushies' Scheme, which [coordinated] Religious Holiday Schools for rural children where there [was] no Catholic school, [was] his creation. In 1925 he founded the Newman Society within the University of Western Australia. He was the editor of the Record, the official organ of the Archdiocese of Perth, from 1928 to 1932. He became a member of the University Senate in 1934 and remained on it for twenty-seven years. In recognition of his services the Senate conferred on him the honorary degree of Doctor of Lettres in 1961. He had a prominent part in establishing the residential College of St Thomas More within the University of Western Australia and [was] a member of its Council. A foundation member of the Australian College of Education, he was elected a Fellow in 1962. In the 1970 New Year's Honours List he received the O. B. E. for his services to Education in the State of Western Australia. Since 1932 he [was] parish priest of St Columbia's, South Perth.'
Such was his influence on Catholic education in Australia, that his work was included in Leadership in Catholic Education, first published in 1990, a year after his death, and continually reprinted into the twenty-first century.
