Mercer County is a county located in the U.S. state of Missouri. As of 2000, the population was 3,757. Its county seat and largest city is Princeton.

Mercer County was organized on February 14, 1845, from part of Grundy County, Missouri. It is named in honor of General Hugh Mercer who fought and died in the American Revolutionary War. The first permanent settlements in what is now Mercer County happened in 1837 when a few families from other parts of Missouri moved to the area. James Parsons, a Tennessee native, is generally considered to be the first permanent settler in the spring of 1837. At that time the land was still included as part of Livingston County and would remain so until 1841 when it became part of newly-created Grundy County. Prior to 1837 the land was used by Native American tribes, primarily the Sac/Fox and Potawatomi, as prime hunting grounds. The occasional temporary hunting village would be established by the tribes but otherwise the land was empty of humans. In 1847 the town of Princeton, named for the battle where General Mercer was killed, was established as the county seat. The northern boundary of the county was in dispute for the first six years of its existence due to the Honey War, a bloodless territorial dispute between Missouri and Iowa. Prior to an 1851 decision by the U.S. Supreme Court, the boundary was 9.5 miles farther north than present day.

Missouri (ⁱ/mɨˈzʊəri/) (nickname The Show Me State) is a U.S. state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. Missouri is the 21st most extensive and the 18th most populous of the 50 United States. Missouri comprises 114 counties and the independent city of St. Louis. The four largest urban areas are St. Louis, Kansas City, Springfield, and Columbia. Missouri's capital is Jefferson City. The land that is now Missouri was acquired from France as part of the Louisiana Purchase and became known as the Missouri Territory. Part of the Territory was admitted into the union as the 24th state on August 10, 1821.

Missouri generally mirrors the demographic, economic and political makeup of America and has long been considered a political bellwether state. With the exceptions of the Presidential elections of 1956 and 2008, Missouri's results have accurately predicted the next President of the United States in every election since 1904. It has both Midwestern and Southern cultural influences, reflecting its history as a border state. It is also a transition between the Eastern and Western United States, as St. Louis is often called the "western-most Eastern city" and Kansas City the "eastern-most Western city". Missouri's geography is highly varied. The northern part of the state lies in dissected till plains while the southern part lies in the Ozark Mountains (a dissected plateau), with the Missouri River dividing the two. The state lies at the intersection of the three greatest rivers of North America, with the confluence of the Mississippi and Missouri Rivers near St. Louis, and the confluence of the Ohio River with the Mississippi north of the Bootheel. The starting points of the Pony Express Trail and Oregon Trail were both in Missouri. The mean center of United States population as of the 2010 Census is at the town of Plato in Texas County, Missouri.

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.