In 1919, Helen Parkhurst relocated to New York City, where she opened her first school on West 74th Street. Eleanor Roosevelt admired the work of Helen Parkhurst and played an important role in expanding the population and resources of the school by promoting a merger between the Todhunter School and Dalton in 1939. Dalton still celebrates many of the school-wide traditions begun by Helen Parkhurst, particularly the Candlelighting Ceremony in the winter, Greek Festival in the spring and Arch Day in June. Over the years, Dalton has gained international recognition for its academic excellence. Schools in The Netherlands, Australia, England, Korea, The Czech Republic, Taiwan and Chile have adopted the Dalton Plan.
Today there are three schools founded on the Dalton Plan in Japan.
Leading educators from universities and public and private schools around the United States and abroad visit Dalton on a regular basis to observe its system of education and to learn more about the school's recognized achievements in the area of technology. Dalton's population and facilities have grown considerably in the last three decades. In 1964, the First Program was moved from 89th Street to a facility of its own at 61 East 91st Street, providing an ideal setting for kindergartners through third graders. In 1978, the First Program expanded to include the adjacent building at 53 East 91st Street, and it was enlarged again in the fall of 1992 to include 63 East 91st Street. One entire floor of this new building was converted into a science center for Dalton's youngest students.