Marshall has devoted his career, for the most part, to domestic and global food security. Starting as Counsel to the Senate Committee on Agriculture he crafted all the nutrition legislation championed by Senators George McGovern (D-SD) and Bob Dole (R-KS). He was also counsel to the Senate Select Committee on Nutrition when it published Dietary Goals for the United States, which has been memorialized as the USDA-HHS Dietary Guidelines and updated every five years. This focus on food security has led to a devotion to sound science, agriculture research, and biotechnology. He now works with the Alliance for a Green Revolution in Africa helping them to grow themselves out of poverty. Other areas of interest have been Native American policy and rural development including forestry. He represents South Dakota State University on the New Beginnings Program for Tribal Students and the Intermountain Forest Association. ​