Frances Hackley, a wealthy widow and leading supporter of the Unitarian movement, decided to give her summer mansion in Tarrytown to a school for boys.

[3] In the fall of 1899, Theodore Chickering Williams and Seaver Buck, the first headmaster and the first master hired, respectively, searched for additional land.

They found a large estate for sale near the grounds of what later became Marymount College, and purchased it with funds from Hackley.

A handbook published in 1920 stated that "Of the two hundred and sixty alumni the great majority have entered Harvard and Yale.

"[4] Clarence Francis, a famous industrialist at the time and special consultant to President Dwight D. Eisenhower, was the commencement speaker at Hackley in 1959, in order to commemorate the school's sixtieth anniversary.

[7] In September 2010, Goodhue Memorial Hall reopened, with the Sternberg Library and computer labs located on the new second floor.

Courses additional to the core curriculum include Creative Writing, History of Western Theater, Seminar in Moral Philosophy, Economics, History of Media & Culture, Art History, Calculus, Finite Mathematics, Statistics, Organic Chemistry, Marine Biology, Ecology, Etymology, Computer Science, Electronic Publishing, Studio Art, Three-Dimensional Sculpture and Design, Architecture and Design, Ceramics, Photography, Filmmaking, Music Theory, Seminar in Music Listening, Acting, Seminar in (music) Composition, and Opera and Jazz, and Student Teaching (where high school students assist in lower school classes).

Newsweek editor Naveed Jamali, who attended the school briefly around 1990, wrote of his lasting memory of the student parking lot: it was filled with "shiny new" expensive cars like Porsches and BMWs.