Cornwall County was a county of the former Province of New York, established on September 5, 1665 from 25,100 square miles (65,000 km2) of land that had been granted to the Duke of York in modern Maine. As established, the grant ran all the way from the St. Lawrence River to the Atlantic Ocean, between the Kennebec and St. Croix rivers, and produced what today is most of Aroostook, Piscataquis, Washington, Hancock, Penobscot, Waldo, Knox, Lincoln, Kennebec, Somerset, and Sagadahoc Counties.