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- Vance C. McCormick, an American politician and prominent businessman; chair of the American delegation at the Treaty of Versailles.
- Scott Mead, an investment banker and a photographer; Former Partner and Managing Director, Goldman Sachs & Company (Graduated 1973)
- Thomas C. Mendenhall (historian), expert of collegiate rowing and former President of Smith College. (graduated 1928)
- Thomas Mesereau, attorney (clients include Robert Blake and Michael Jackson) (graduated 1969)
- John Marsh, first school teacher in old NW Territory and wrote first Dakota Indian dictionary and an early advocate for California joining the American Union (graduated 1818)
- Othniel Charles Marsh, first professor of paleontology at Yale University (graduated 1856)
- Laurel Massé, founding member of Manhattan Transfer, (expelled 1969)
- Charles A. Meyer, Assistant Secretary of State (graduated 1935)
- Barry R. McCaffrey, teacher of national securities studies at West Point (graduated 1960)
- Priscilla Martel, restauranteur and food expert (graduated 1974)
- Jonathan Meath, award–winning children's TV producer and famous Santa Claus (graduated 1974)
- Marvin Minsky, noted authority on artificial intelligence, robotics and computers.
- William H. Moody, United States Supreme Court justice (graduated 1871)
- Emily Moore, American poet (graduated 1995)
- Samuel Morse, inventor of telegraph and Morse Code (graduated 1805)
- Paul Monette, author and activist (graduated 1963)
- Lachlan Murdoch, son of media mogul Rupert Murdoch, former Executive Director of News Corporation (graduated 1990)
- Charles B. G. Murphy, Writer and Philanthropist Yale (graduated 1923) Honored with the Charles B. G. Murphy professorship
- Patrick O. Murphy, Mayor of Lowell, Massachusetts
Read more about this topic: List Of Phillips Academy Alumni