Ernest Mahan was born July 25, 1898, in West Baden Springs, Indiana, to Andrew and Rena Mahan. On April 22, 1923, Mahan married Maurine Mendenhall at her parents' home in Yorktown, Indiana. Thereafter, Mahan attended the University of Indiana and received a bachelor's degree and master's degree. Transferring to the University of Wisconsin, Mahan earned a doctorate degree in United States history in 1930.

From 1917-1920 Mahan taught at the elementary school in West Baden Springs. He served as principal of the Senior High School (1920-1921) in Galveston, Indiana, and one year each in the same position at Mount Summit, Indiana, and Williamsburg, Indiana. During 1924-1928, Mahan worked as principal of the Senior High School in Centerville, Indiana. After teaching one summer session in the State Teachers College at Terre Haute, Indiana, in 1930 Mahan was appointed as assistant professor of social science at the Kansas State Teachers College (now Pittsburg State University).

Mahan was appointed to the rank of associate professor in 1931, professor in 1932, and in 1939, he became head of the department. In 1945 Mahan became dean of instruction and head of the Department of Social Science. He resigned as dean of instruction in 1959, but he continued to teach full-time until his retirement in 1968, and thereafter, he served as an adjunct for several years.

During Mahan's life he was a member and elder of the Pittsburg First Presbyterian Church. His professional memberships included the Kiwanis Club, Phi Alpha Theta fraternity, the Kansas State Historical Society, the National Education Association, the American Association of School Administrators, the Mississippi Valley Historical Association, the Kansas History Teachers Association, the Kansas Educators Club, and the American Association of University Professors. Mahan contributed numerous articles to the Mississippi Valley Historical Review, Dictionary of American History, The Historian, and The Educational Leader. Ernest Mahan died in Kansas on September 14, 1985.