High Quality Content by WIKIPEDIA articles! Union County is a county located in the U.S. state of New Jersey. As of the 2000 Census, the population was 522,541. It is part of the New York Metropolitan Area. Its county seat is Elizabeth. Union County ranks 92nd among the highest-income counties in the United States. It also ranks 74th in the United States by personal per-capita income. With more than 5,000 persons per square mile on average, Union County is one of the most densely populated counties in America. According to the U.S. Census Bureau, the county has a total area of 105 square miles (273 km²), of which, 103 square miles (268 km²) of it is land and 2 square miles (6 km²) of it (2.06%) is water. Much of Union County is relatively flat and low-lying. Only in the northwestern corner does any significant relief appear as the Watchung Mountains cross the county. It is there that highest elevations, two areas approximately 560 feet (171 m) above sea level, are found in Berkeley Heights. The lowest elevation is sea level along the eastern shore.