Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton, the state capital. It is officially part of both the New York metropolitan area and the Trenton-Ewing Metropolitan Statistical Area. Unofficially, it is considered part of the Delaware Valley (Philadelphia) Metropolitan Area.
The county is named for Continental Army General Hugh Mercer, who died as a result of wounds received at the Battle of Princeton on January 3, 1777. The Mercer Oak, against which the dying general rested as his men continued to fight, appears on the county seal. As of the 2010 United States Census, the population was 366,513, an increase of 15,752 (4.5%) from the 350,761 enumerated in the 2000 Census, making it the 12th-most populous county in the state. Mercer County ranks 80th among the highest-income counties in the United States. The Bureau of Economic Analysis ranked the county as having the 78th-highest per capita income of all 3,113 counties in the United States (and the sixth-highest in New Jersey) as of 2009.
Mercer County is home to Princeton University, Princeton Theological Seminary, the Institute for Advanced Study, Rider University, The College of New Jersey, Thomas Edison State College and Mercer County Community College.
Founded February 22, 1838, from portions of surrounding counties, Mercer County has a historical impact that reaches back to the pivotal battles of the American Revolutionary War. On the night of December 25–26, 1776, General George Washington led American forces across the Delaware River to attack the Hessian barracks in Trenton on the morning of December 26. Following the battle, Washington crossed back to Pennsylvania. He crossed a third time in a surprise attack on the forces of General Charles Cornwallis at the Second Battle of Trenton on January 2, 1777 and at the Battle of Princeton on January 3. The successful attacks built morale among the pro-independence colonists.
Mercer County also has the distinction of being the famed landing spot for a fictional Martian invasion of the United States. In 1938, in what has become one of the most famous American radio plays of all time, Orson Welles acted out his The War of the Worlds invasion. His imaginary aliens first "landed" at what is now West Windsor Township. A commemorative monument is erected at Grover's Mill park.