C argill MacMillan III is an American billionaire and an heir to the Cargill fortune, one of the largest and most significant family fortunes in the world. He is one of four brothers who are great-great-grandsons of William W. Cargill, who founded the agribusiness giant in 1865. Cargill is the largest private company in the United States, with vast operations in commodity trading, food processing, and agricultural services.

The Cargill and MacMillan families have controlled the company for over 150 years, and their collective wealth is immense. Like the other heirs of his generation, Cargill MacMillan III is not involved in the day-to-day management of the company, which is run by professional, non-family executives. His wealth, and that of his brothers William, John, and Gwendolyn, is derived from his inherited stake in the enduring and sprawling family enterprise. He maintains a very low public profile.