Duncan L. MacMillan is an American businessman, mathematician, and philanthropist, best known as one of the four co-founders of Bloomberg L.P., the global financial data and media giant. Born in 1937, his career is marked by a blend of military discipline, technical expertise, and financial acumen.

He served four years in the United States Marine Corps, instilling a rigorous organizational discipline. He graduated with a Bachelor of Science degree in Mathematics from Rutgers University in 1966. His financial career began at Bankers Trust in 1967, where he met his wife, Nancy. He later worked at Salomon Brothers alongside Michael Bloomberg, Charles Zegar, and Thomas Secunda—the trio who would become his co-founders.

Duncan MacMillan co-founded Innovative Market Systems (later renamed Bloomberg L.P.) in 1982. While Michael Bloomberg was the visionary, MacMillan was instrumental in the technical execution: he is credited with designing many of the computer systems the company uses to disseminate critical data and communicate information to subscribers. He served as the firm's expert on customer needs, ensuring the proprietary Bloomberg Terminal was user-friendly and highly functional.

His wealth is tied to his founding equity stake in the privately held Bloomberg L.P., which became the indispensable information service for the global financial market. He and his wife, Nancy, are prominent philanthropists, having signed The Giving Pledge, committing the majority of their wealth to charity. His structural contribution is the successful development of the core software and systems that define modern financial trading.

Duncan MacMillan is a dedicated philanthropist, focusing his giving on cancer research, education, and health, often supporting his alma mater and local institutions (like the Rutgers Cancer Institute of New Jersey). He and his wife were the first to endow two academic chairs at Rutgers University (in genetics and the life sciences).

His structural social impact is tied to Bloomberg's role in standardizing financial data flow globally. His commitment to education ensures the continuous advancement of scientific and technical research.

Duncan MacMillan maintains the professional, disciplined style of a financial executive and former military officer. His attire is consistently formal and high-quality, favoring tailored suits. His aesthetic is one of stability and composed technical authority.

Residing in Princeton, New Jersey, his luxury is the immense security and autonomy derived from his multi-billion dollar fortune. His life is dedicated to the technical precision of financial data systems and his profound commitment to educational and medical philanthropy.