Millstone Township is a township in western Monmouth County, in the U.S. state of New Jersey. The township is located within the Raritan Valley region and is a part of the New York metropolitan area. As of the 2020 United States census, the township's population was 10,376, a decrease of 190 from the 2010 census count of 10,566, which in turn reflected …Millstone Township is a township in western Monmouth County, in the U.S. state of New Jersey. The township is located within the Raritan Valley region and is a part of the New York metropolitan area. As of the 2020 United States census, the township's population was 10,376, a decrease of 190 from the 2010 census count of 10,566, which in turn reflected an increase of 1,596 from the 8,970 counted in the 2000 census.