Mercer County is the name of several counties in the United States:
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 2014, Mercer County's Census-estimated population was 371,537, an increase of 1.4% from the 2010 United States Census, when its population was enumerated at 366,513, in turn 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.
Mercer County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 116,638. Its county seat is Mercer, and its largest city is Hermitage. The county was created in 1800 and later organized in 1803.
Mercer County is included in the Youngstown-Warren-Boardman, OH-PA Metropolitan Statistical Area.
According to the U.S. Census Bureau, the county has a total area of 683 square miles (1,770 km2), of which 673 square miles (1,740 km2) is land and 10 square miles (26 km2) (1.5%) is water.
As of the census of 2000, there were 120,293 people, 46,712 households, and 32,371 families residing in the county. The population density was 179 people per square mile (69/km²). There were 49,859 housing units at an average density of 74 per square mile (29/km²). The racial makeup of the county was 93.13% White, 5.25% Black or African American, 0.11% Native American, 0.40% Asian, 0.02% Pacific Islander, 0.17% from other races, and 0.91% from two or more races. 0.67% of the population were Hispanic or Latino of any race. 24.5% were of German, 12.0% Italian, 10.5% Irish, 9.3% English and 6.8% American ancestry according to Census 2000.