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- Storm Large, musician. Her father Henry spent 45 years teaching history and coaching football and baseball at the school before his retirement in 2010.
- Frederick Lippitt, Rhode Island lawyer, politician, public servant and philanthropist.
- Henry Demarest Lloyd, 19th century muck-raking reporter; "the father of investigative journalism"
- Christian Lorentzen, editor & critic
- Robert Lowell, poet
Read more about this topic: List Of St. Mark's School (Massachusetts) Alumni