[1] He was backed by a coterie of wealthy Bostonians, including his brother Robert (the managing partner of the Kidder, Peabody investment bank), Francis Cabot Lowell, Norwood Penrose Hallowell, William Cameron Forbes, Henry Lee Higginson, and Charles Jackson Paine, the latter of whom donated the land for the school's campus.

[17] Middlesex was founded to "meet the needs of the large body of Unitarian parents who [we]re not wholly satisfied to send their boys to the so-called [Episcopal] Church schools" like St. Paul's, St. George's, St. Mark's, and Groton.

[23] Today, Middlesex's chapel hosts secular school meetings on Wednesdays, and there are no regular religious services.

[25] Harvard president Charles Eliot (the namesake of a Middlesex administrative building) was an early backer of the school,[26] and Harvard dean LeBaron Russell Briggs was a founding member of the Middlesex board of trustees; both of them were Unitarians.

[27] (In fact, at the time Middlesex was founded, Eliot's son Samuel was president of the American Unitarian Association.

[30] In 1967, Middlesex placed more students at Harvard, Yale, and Princeton (as a percentage of the graduating class) than all but one New England boarding school.

[32] Middlesex is the only nonsectarian member of St. Grottlesex, an informal grouping of five schools historically associated with upper-class White Anglo-Saxon Protestant culture.

[33] Its combination of Eastern Establishment prestige and religious permissiveness made it attractive to Jewish, Catholic, and Southern Baptist families who wanted their children to attend an elite boarding school, but not an Episcopalian one.

[41] (Although Schiff's business partner Otto Kahn did send his son to Groton, Kahn—unlike Schiff—was not religiously observant.

Due to its financial reliance on white Southern families, Middlesex was one of the last major New England boarding schools to admit black students, doing so in 1964.

[56] Middlesex's primary athletic rival is the St. George's School in Middletown, Rhode Island.

[58] Students may help clean up a soup kitchen at Open Table (weekly), serve food and clean at a food pantry at Cor Unum (on long weekends), talk to people at a home for the elderly at Walden House (weekly), visit the elderly at Sunday Visits (special schedule), and help small children learn to skate at Gazebo (special schedule).

Every summer, the school sponsors a community service trip to the Linawo Children's Home in South Africa, where students tour the surrounding area, learn about South African culture and history, and assist in the operation of the shelter.