High Quality Content by WIKIPEDIA articles! Union County is a county located in the U.S. state of New Jersey. As of the 2010 Census, the population was 536,499. It is part of the New York Metropolitan Area. Its county seat is Elizabeth. Union County ranks 93rd among the highest-income counties in the United States. It also ranks 74th in the United States by personal per-capita income. A study by Forbes.com determined Union County pays the second-highest property taxes of all counties in the nation. With more than 5,000 persons per square mile on average, Union County is one of the most densely populated counties in America.