Mrs. Caleb Brewster Hackley, a wealthy widow and leading supporter of the Unitarian movement, decided to give her summer mansion in Tarrytown to a charity. She resided in New York City, and was a member of the Church of the Messiah. Hackley was friends with the church's minister, who proposed using the mansion as a school for boys. In the winter of 1898-1899, she met with Dr. Samuel Eliot of Boston, who later became the President of the American Unitarian Association, and several other prominent Unitarians. Mrs. Hackley liked the idea of a college preparatory school to serve the Unitarian community and any families interested in a liberal religious environment and wanted to use her home for this purpose. She provided substantial funding to refurbish the mansion for school purposes and to operate the school for several years. In the spring of 1899, a Board of Trustees was formed and very shortly thereafter it selected its first headmaster. The first students arrived in the autumn of 1899 and resided in the Hackley home, today called Hackley Hall. In what became a part of Hackley tradition, generations of Hackley students will swear that Mrs. Hackley's ghost haunts the third floor of the Upper School building. Hackley School celebrates Mrs. Hackley's birthday, October 27th, as "Founders Day" to this day.
In September 2015, Hackley announced the appointment of Michael C. Wirtz as Hackley's 12th head of school. Mr. Wirtz began his tenure July 1, 2016, succeeding Walter C. Johnson, who served as headmaster from 1995-2016.