Following his time in the Navy, Dr. Marshall spent time training in general surgery with Dr. Warren H. Cole and as his research assistant and an instructor in surgery. He then went to the University of Louisville as an Assistant Professor of Surgery. He returned to Peoria in 1964 to join the Peoria Surgical Group. In the early 1970s he was asked to work on the development and organization of the surgery department for the new Peoria School of Medicine. He was the first chair of the Department of Surgery and led the department from 1972 to 1982.