Following his graduation at age 16 from St. Cecilia High School in Hastings, Mullen went on to study at St. Benedict College in Atchinson, Kansas.
[3] Upon completing his PhD at the University of California-Berkeley, Mullen accepted a job at then-Montana State College and moved to Bozeman, Montana.
[3][5] Mullen participated in National Humanities summer grants at Harvard, UC Santa Barbara, and West Point.
[3][6] In 1993, Mullen co-authored In the People's Interest: A Centennial History of Montana State University with Jeffrey J. Safford and Robert Rydell.
[8] Mullens was also deeply involved in local organizations, including holding the position of President for the Headwaters Chapter of the Lewis and Clark Trail Heritage Foundation, a member of the Quest for Knowledge Club, and a member of the AROHE (Association of Retirement Organizations in Higher Education).