Upon August Belmont's death in 1924, the lands were passed to his widow, Eleanor Robson Belmont, a famous American actress and playwright. Early in 1925 she sold all but a small portion of the estate to Cadman H. Frederick who subdivided most of it into lots. The remaining 158 acres, including the lake, the mansion and the main farm buildings were sold to the State of New York. In 1927 the State purchased land that had been retained by Mrs. Belmont. Since that time, further acquisitions have brought the total acreage of Belmont Lake State Park to 459.
The family mansion was used for a time as Long Island State Park Commission administration headquarters. In 1935 it was razed to make way for the present office building which was erected on the site of the Belmont estate's Episcopal Chapel. In addition to the office building, central maintenance and shop buildings are located here.