S
- Peter Saccio, Shakespeare scholar and author, educator
- Stephen "Laddie" Sanford, international poloist
- John Sargent, former president and CEO of Doubleday and Company publisher
- Eugene Lytton Scott, American tennis player; member of International Tennis Hall of Fame, founder of the magazine Tennis Week
- John Sculley, former president of PepsiCo and former CEO of Apple Computer
- John W. Sears, Massachusetts Metropolitan District Commissioner
- Mason Sears, member of the Massachusetts General Court, Chairman of the Massachusetts Republican Party, and United States' Representative to United Nations Trusteeship Council
- John Simpkins, Representative from Massachusetts, 1895–1898
- Nik Stauskas, Basketball Player
- Tyler Simpson, Portfolio Manager
Read more about this topic: List Of St. Mark's School (Massachusetts) Alumni