Whitney MacMillan was born on September 25, 1929. He is the great-grandson of william W. Cargill, the founder of Cargill. His Father was Cargill MacMillan, Sr.. He has one brother, Cargill MacMillan, Jr. (1927-2011), and one sister, Pauline MacMillan Keinath. He graduated from Yale University.

He served as CEO of Cargill from 1976 to 1995. He was the last member of the Cargill family to serve as CEO. During his tenure, Cargill went from $10 billion to $33 billion over ten years. By the 1980s, it became the world's largest grain company, outstripping its European rivals.

The Whitney and Betty MacMillan Center for International and Area Studies at Yale is named for him. The Painter Michael Shane Neal has painted his portrait. In 2002, he received an honorary doctorate from Montana State University-Bozeman. He is an Executive Fellow and Teacher at the University of St. Thomas Graduate School of Business in Minnesota.

He is married to Betty MacMillan, and they have two children. He lives in Minneapolis, Minnesota. As of August 2014, he was worth an estimated US$4.8 billion.

He has served on the Board of Directors of the International Peace Institute, the Salzburg Global Seminar, the Rural Development Institute, the Trilateral Commission, Hubert H. Humphrey School of Public Affairs at the University of Minnesota, the Ruckelshaus Institute, the Burton K. Wheeler Center for Public Policy in Bozeman, Montana and Yale University's President’s Council on International Activities. He serves on the National Advisory Board of the Museum of the Rockies in Bozeman, MT. He is a member of the National Academy of Sciences Board on Agriculture and Natural Resources, the EastWest Institute, Care International, Mayo Clinic and the Council on Foreign Relations.