Horace Mann School was founded by Nicholas Murray Butler in 1887 as a coeducational experimental and developmental unit of Teachers College, Columbia University. In 1947, it became an independent day school for boys in grades seven through twelve. The reestablishment of coeducation was accomplished through mergers with the New York School for Nursery Years in 1968, the Barnard School in 1972, and the enrollment of girls in the high school beginning in 1975. In 1887, a full year's tuition for a high school senior was $150. The School’s founding fathers named the school after Horace Mann (1796-1859). Horace Mann was a lawyer who served in the Massachusetts State Legislature. He was the first Secretary of the Massachusetts Board of Education, a Member of the United States House of Representatives, and President of Antioch College. He used each of his positions to proclaim that every person, regardless of their background, should receive a public education based on the principles and practices of a free society. He also proclaimed that slaves should be free, women should vote and the mentally ill should be cared for. Although Horace Mann’s ideas were revolutionary, he did play a leading role in establishing the elementary school system in the United States. The Barnard School was founded. Teachers College and Model School founded at No. 9 University Place. Nicholas Murray Butler leaves the Presidency of Teachers College, and Dr. William Hervey becomes President. Horace Mann School and Teachers College move into a single building on 120th Street. Virgil Prettyman becomes Principal of Horace Mann School. Horace Mann School moved into its own building at 120th Street and Broadway in Morningside Heights. The School was across the street from the former Bloomingdale Insane Asylum which was being used as a students’ dormitory at Teachers College. By this time, however, Horace Mann was becoming less of an experimental school for the students of Teachers College to try out their new ideas, and more of a well-recognized school in its own right. Teachers College eventually created the Lincoln School as a new co-educational school in which to practice their experimental teaching methods, leaving Horace Mann more and more independent. Teachers College purchased lots on 246th Street in the Fieldston section of the Bronx. Alumni Field opened there as an athletic facility. The property is reported to have cost $20,001, which was paid for by Mr. Prettyman. Horace Mann School for Boys opened on 246th Street. Henry Carr Pearson became the Headmaster of the Horace Mann High School for Girls, still on 120th Street. Charles Carpenter Tillinghast was appointed Headmaster. Prettyman Gymnasium and Pool opened. Horace Mann High School for Girls merged with the Lincoln School. Horace Mann-Lincoln School closed. Horace Mann School for Boys obtained complete financial and administrative independence from Teachers College. The new Board of Trustees received a provisional charter changing the School's name back to Horace Mann School. Dr. Mitchell Gratwick becomes the third Headmaster. Horace Mann's permanent charter from the New York Board of Regents was granted. The New York School for Nursery Years was founded.

Pforzheimer Hall was built, named after Mr. Carl H. Pforzheimer, who was chairman of the Board of Trustees. Alfred Gross Hall, housing the Van Alstyne Auditorium and new cafeteria, opened. The old auditorium became the Theresa H. Loeb Library. The fourth floor of the newly-renamed Tillinghast Hall was converted from a gymnasium into classroom space. The John Dorr Nature Laboratory, 85 acres of land in rural Washington Depot, Connecticut, was left to the school by John Dorr, an inventor and a neighbor of Headmaster Gratwick. Robert Thomason was named Headmaster. Horace Mann established the Horace Mann School for Nursery Years through a merger with the New York School for Nursery Years (which had been founded in 1954 and housed in Andrew Carnegie's old carriage house on 90th Street on the Upper East Side). Harry Williams was named Acting Headmaster. R. Inslee Clark became Headmaster. Horace Mann merged with the Barnard School. Horace Mann Elementary school moved into the former Barnard facilities. Girls were admitted to Horace Mann High School after almost 60 years of separate schooling; the Gratwick science wing was added onto Pforzheimer Hall. Horace Mann celebrated its centennial anniversary. Phillip Foote was named Headmaster. Dr. Eileen Mullady was appointed as the first female Head of School. Rose Hall, home to the Horace Mann Middle Division, and Fisher Hall, home to the Arts and Dining, opened. The newly renovated Tillinghast Hall reopened, and Mullady Hall, home to the Katz Library and Alfred Gross Theater, opened. Dr. Thomas M. Kelly was appointed as the ninth Head of School. Newly built, LEED certified, lodge and barn/classroom opened at the now-275 acre John Dorr Nature Laboratory. Horace Mann School reaccredited by the New York State Association of Independent Schools (NYSAIS), following a year long self-study and a visit by a 19-person committee of educators. Lutnick Hall opens housing new Upper Division science laboratories and classrooms, and the Mindich Family Campus Center featuring the Mignone Independent Research Labs, the Olshan Great Room, a Student Publications Suite and meeting rooms and the Mindich Center for Community Values and Action Program. The Jefflrey H. Loria Family Aquatic Center and Friedman Hall open, and Prettyman Hall reopens with the renovated Richard A. Friedman '75 and Steven M. Friedman '72 Gymnasiums and the new Simon Family Fitness Center. Renovated and reimagined Pforzheimer and Rose Halls greeted students in the fall. State of the art and developmentally appropriate science facilities, classrooms, and storage welcomed not only students, but a new full-year science elective program for our eighth graders. New spaces were created for the Middle Division Deans and Guidance & Counseling program, the Middle Division Reading Room was moved from Katz Library to Pforzheimer Hall and the administrative offices were relocated to the front of Pforzheimer Hall. New and more efficient mechanicals were installed in both Pforzheimer and Rose Halls.