Prior to joining the academic world, MacMillan was a chemical engineer and gained experience in gold and uranium mining, chemical and explosives manufacturing, oil refining, and soap and food production; he was also a scientist at the South African Atomic Energy Board. MacMillan has been a director of several companies in the travel, import/export, and pharmaceutical industries. He also has extensive consulting experience with such companies as Air Products, DuPont, General Electric, GTE, IBM, Citibank, Chubb & Son, American Re-Insurance, Texas Instruments, KPMG, Hewlett-Packard, Intel, Fluor Daniel, Matsushita (Japan), Olympus (Japan), and L. G. Group (Korea).

MacMillan’s articles have appeared in the Harvard Business Review, Sloan Management Review, Administrative Science Quarterly, Academy of Management Journal, Academy of Management Review, Academy of Management Executive, Management Science, Strategic Management Journal, and many other publications.