Mercer County is located in the U.S. state of New Jersey. Its county seat is Trenton. Since 2000, Mercer County has been considered part of the New Jersey metropolitan area. Mercer County ranks 79th among the highest-income counties in the United States with a per capita income of $27,914. Mercer County has a total area of approximately 229 square miles of which 99% is land and 1% water. As of the 2000 census, there were 350,761 people, 125,807 households, and 86,303 families residing in Mercer County, NJ.