MacMillan, 84, who retired from Cargill in 1988, was among the last generation of family members who played an active role in the Minnesota-based company, the Minneapolis Star Tribune reported Wednesday.
His wealth was estimated by Fortune Magazine as $2.6 billion making him one of the richest longtime residents of Minnesota. In 1990, he and his wife Donna moved to a home in the Palm Springs area of California where he died Monday.
A great-grandson of W.W. Cargill, MacMillan served in the U.S. Air Force and graduated from Yale University.
He started working for Cargill in 1950 as a trainee and eventually became a senior executive.
He served the company for 38 years before retiring.
In addition to his wife, MacMillan is survived by six children, 24 grandchildren, two great-grandchildren, a brother and two sisters.