Mercer County is a county located in the U.S. state of New Jersey. Its county seat is Trenton, the state capital. The county is part of the Trenton, NJ Metropolitan Statistical Area and is considered part of the New York metropolitan area by the United States Census Bureau, but also directly borders the Philadelphia metropolitan area and is included within the Federal Communications Commission's Philadelphia Designated Market Area. 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, retaining its position as the 12th-most populous county in the state. Mercer County stands among the highest-income counties in the United States, with the Bureau of Economic Analysis having 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.
The county was formed by an Act of the New Jersey Legislature on February 22, 1838, from portions of Burlington County (including Nottingham Township), Hunterdon County (including Ewing Township, Lawrence Township, Trenton City and portions of Hopewell Township), and Middlesex County (including West Windsor Township and portions of East Windsor Township). The old Keith Line bisects the county and is the boundary between municipalities that previously had been separated into West Jersey and East Jersey. It was 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 and stood for 250 years until it collapsed in 2000.
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 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.
There were 27 Mercer County residents killed during the September 11, 2001 terrorist attacks in Lower Manhattan. A long steel beam weighing one ton was given to the county by the Port Authority of New York and New Jersey in March 2011 and is now displayed at Mercer County Park.