Marshall joined the University of Iowa College of Dentistry in 1991. She is a registered dietician and a professor in the Department of Preventive and Community Dentistry. In 2010, she was appointed director of experiential learning.

Marshall coordinates and teaches the nutrition curriculum within the College of Dentistry. She also directs the experiential course for first- and second-year dental students.

Marshall's research interests focus on nutrition, oral health and systemic health. Current research efforts include diet, growth, and oral health in young children, as well as nutrition and oral health in the elderly.