Mullin served as a professor of medicine at the University of Chicago from 1939 to 1951, and as dean of the faculties and professor of physiology at Chicago Medical School from 1951 to 1954.
Mullin served as a professor of medicine at the University of Chicago from 1939 to 1951, as well as serving as dean of students there for part of this time. He also served as dean of the faculties and professor of physiology at Chicago Medical School from 1951 to 1954.