During the early 1780s, Cooper became a storekeeper in Burlington, New Jersey, located along the Delaware River. By the end of the decade, he was a successful land speculator and wealthy frontier developer in what is now Otsego County, New York. Soon after the conclusion of the Revolutionary War, he acquired a tract of land several thousand acres in extent within the borders of New York state and lying along the head waters of the Susquehanna River at Otsego Lake. He founded the Village of Otsego at the foot of the lake in 1786, creating a traditional plan for the village inspired by that of Burlington. He moved his family there, arriving on November 10, 1790. The judge and other investors also founded De Kalb, New York, near the east end of Lake Ontario and the St. Lawrence River, in 1803, platting approximately 64,000 acres and selling the parcels on. His brother James took care of the holdings, which were tied up in litigation for years, both before and after the death of Judge Cooper in 1809. The Cooper family holdings were all gone by 1817. In 1852 a village was named Cooper's Falls north of De Kalb. James stayed in the area, and his son William grew up in De Kalb and was a carpenter. The existing De Kalb Historical Society building was built by the judge's nephew, William, and some of his descendants may still live in the area.