Leeds and Grenville The United Counties of, is located in Southeastern Ontario, Canada. The population as of the 1991 census was 96,606. The United Counties have an area of 3,390 sq km (1,309 sq mi). Leeds and Grenville front on the St. Lawrence River opposite the State of New York at a mid point between Cornwall in the East and Kingston in the West. Brockville is the county seat for the United Counties and its only City in Leeds County. Prescott is the principal town of Grenville County.