W illiam MacMillan is an American heir to the Cargill fortune, one of the largest and most significant family fortunes in the world. He is a great-grandson of John H. MacMillan, Sr., who became the leader of the company after marrying Edna Cargill, the daughter of the company's founder, William Wallace Cargill. This union cemented the two families' control over the agribusiness giant for over a century.

Cargill is the largest privately held corporation in the United States, with vast global operations in commodity trading, food processing, agricultural services, and risk management. The immense, multi-generational wealth of the family is derived from their estimated 90% ownership stake in this sprawling enterprise. William MacMillan, like most of his relatives, is not involved in the executive management of the professionally run company. He has maintained a life away from the public eye, and his fortune is managed through family trusts and holding companies.