Westminster School was founded by William Lee Cushing in 1888 as a boys’ school in Dobbs Ferry, New York.
In 1900, as enrollment increased, Mr. Cushing moved the school to its current location in Simsbury, Connecticut. The move to Simsbury provided more land, which had been donated through a trustee of the school, Arthur M. Dodge, a member of an old Hartford family. Williams Hill (the site of the school) offered more than 230 acres (0.93 km²) with commanding views of the Farmington River. The Simsbury location also provided train service for students to New York and Boston, a boon to families from those areas.
A graduate of Yale University and a firm believer in the traditional form of English boarding school education, Mr. Cushing was strongly influenced by the Reverend Edward Thring the headmaster of Uppingham School in England.
In the early 1970s, Westminster School opened its doors to day students. In 1971, girls were admitted for the first time as day students and in 1977 as boarding students. Like many boarding schools, Westminster faced difficult times in the 1970s as it competed for a shrinking pool of boarding students led by Headmaster Donald Werner (appointed in 1972). When Werner retired after nearly twenty-five years, he left a thriving school for successor Graham Cole. With Cole's retirement in 2010, Westminster has appointed William V.N. ("Bill") Philip as its eighth Headmaster. Philip has ascended to the top job after a 26-year career at Westminster as a teacher, coach, dorm-master, college counselor, and most recently Associate and Assistant Headmaster
 
Many students' favorite tradition is stickball, a game in which teams made up of dormitory floors and day student teams compete in a baseball-like game on the quad and athletic fields in the late spring when the days lengthen and the temperature rises. Each floor must make its own bat, usually a hockey or lacrosse stick that has been cut, or a wooden dowel of a large diameter. Generally the stickball "season" will culminate in a single-elimination tournament to crown the Hill Stickball champion.
 
Theatre
Theater has always been an integral part of the Westminster experience dating back to when Headmaster Cushing's son wrote and directed productions each winter before he launched a very successful career as a Broadway producer. The theater program at Westminster offers students the opportunity to experience all aspects of theatrical performance and production in the theoretical setting of the classroom and the practical arena of our department productions.
Each year the Theater program produces three productions in the state of the art Werner Centennial Theater. One dramatic production spanning the varied genre of Western theater. A musical production, and the student directed one-act plays, which offer our advanced students the opportunity to direct a one -act play as part of an evening of student directed plays. Each of these productions offers many opportunities for student involvement and leadership both on stage and backstage.
 
Alumni
Notable alumni include:

• Lake Bell '97, actress
• Eric Boguniecki, NHL Hockey Player
• Ethan Brooks '91, NFL Football Player
• Joy Bryant '92, actress
• Jack Du Brul, writer.
• Andrew Firestone, The Bachelor TV Series
• Peter Fonda, actor
• Graham Gund '59, architect.
• Alec Musser, actor
• Ben Smith '06, NHL Hockey Player.
• Wellesley Wild '90, writer and executive producer of Family Guy.
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• A seventh season episode (713) of the MTV show Made was filmed over three June days on Westminster campus. It was the episode where the spoiled girl, Katrina, tried to become a soccer player.
• In the 1990s the Centennial Theatre Festival was held in the summer on campus.
• Cole Porter attended many early Westminster Dramatic Productions at Simsbury's Casino.
• Before becoming coed, Westminster performed their Dramatic Productions with girls from The Ethel Walker School including Oscar-nominated actress Sigourney Weaver.