Campbell joined MIT as a research associate, then worked for 10 years in the Virginia Tech Department of Biochemistry and Nutrition, before returning to Cornell in 1975 to join its Division of Nutritional Sciences. He has worked as a senior science adviser to the American Institute for Cancer Research, and sits on the advisory board of the Physicians Committee for Responsible Medicine. He is known in particular for research, derived in part from the China study, that appears to link the consumption of animal protein with the development of cancer and heart disease. He argues that casein, a protein found in milk from mammals, is "the most significant carcinogen we consume".