In 1890, Ruth Livingston Mills, the great-grand daughter of Morgan Lewis and the mother of Ogden L. Mills, Secretary of the Treasury, inherited the estate. Not long afterwards, Ruth Livingston Mills and her husband, the financier and philanthropist Ogden Mills, commissioned a remodeling and enlargement of the mansion to the architectural firm of McKim, Mead, and White; the principal architect was Stanford White. Work started in 1895 and when completed in the following year, he had turned the house into a Beaux-Arts mansion with 65 rooms, 14 bathrooms, and 23 fireplaces. The work is reported to have cost about $350,000. As part of the process, a coal-powered electricity plant was constructed near the Hudson River to supply the electric lights in the mansion, which was also equipped with central heating. The mansion was used as a residence and for entertaining by the family; they owned five homes in total, and stayed at Staatsburgh mainly during the fall season between mid-September and the Christmas holidays. During this time, they hosted house parties, balls, and dinners. At the remainder of the year, the family stayed in one of their other residences in New York City, Paris, Newport, Rhode Island, and California. Before her death, Ruth Livingston Mills began acquiring lands surrounding the estate. After her death in 1920, Ogden Mills continued to purchase the surrounding property, enlarging the acreage of the property to total over , the size of the mansion grounds at the time that it was given to the state of New York. In 1929, the property was passed on to their son, Ogden Livingston Mills and upon his death in 1937, on to his sister Gladys Mills Phipps.