James H. McMahon was born in Ireland July 10, 1830. About 1848, McMahon immigrated to the United States. He settled in Kentucky, where he worked as a carpenter while studying medicine.
By 1860, Dr. McMahon was practicing medicine in Bardstown, Kentucky. On September 29, 1864, he was mustered into the 54th Kentucky Infantry as an assistant surgeon at Louisville, Kentucky by special request of the Governor of Kentucky . McMahon was mustered out of the Union Army September 1, 1865 in Louisville.