Acting as his own architect, Sir William Johnson built a large three-story stone house about a mile west of his first home. Following the Georgian style of architecture, it was finished in 1749 and was to be called Fort Johnson. Along with the house, a grist mill and out buildings were on the property which were drawn on a plan in 1759. Structures flanked the house on the front and were fortified as a means of defense from attack by the French. Soon Johnson was ordering new furnishings from England and locally made pieces to enhance his new home. William Johnson had constructed a larger house for himself and his family. He and Catherine Weissenberg had three children - Ann, John and Mary. Catherine died in 1759 while at Fort Johnson. After her death, Molly Brant, sister of the Indian leader Joseph Brant, managed the household at Fort Johnson and later at Johnson Hall; she and Johnson had eight children. With Sir William's removal to Johnson Hall, Fort Johnson was given to his son John. John occupied the Fort until 1774, when upon Sir Willam's death, John inherited the baronetcy and moved to Johnson Hall.
After Sir John and Lady Mary moved to Johnson Hall, Fort Johnson was used by other members of William's family. But the Fort did not remain in the Johnson family for long, 1775 saw the outbreak of the American Revolution and Sir John Johnson's decision to remain loyal to the British Crown. Sir John was attainted a traitor, and he and his family fled to Canada. All Johnson property Was confiscated, with Fort Johnson seized by the Tryon County Committee of Sequestration in 1777. In Canada, Sir John raised the King's Royal Regiment of New York. Known as Johnson's Royal Greens, they raided the Mohawk and Schoharie Valleys, returning to Canada after the Revolution. Fort Johnson and all the furnishings were sold at auction.
Throughout the nineteenth century the house passed through many hands. Then in 1905, the Fort was again for sale. it was purchased by Brig. Gen. John Watts De Peyster, a descendant of Mary Watts, and given to the Montgomery County Historical Society.
Source: Fort Johnson brochure