The county is named for Continental Army General Hugh Mercer, who died at the Battle of Princeton in 1777. The Mercer Oak, against which the dying general rested as his men continued to fight, appears on the county seal. As of the United States 2000 Census, the population was 350,761, estimated to have risen to 367,605 as of 2006. Mercer County ranks 79th among the highest-income counties in the United States. It also is ranked 57th in the United States by per capita income.
Mercer county is home to Princeton University, Institute for Advanced Study, Rider University, The College of New Jersey, Thomas Edison State College and Mercer County Community College.
As of the census of 2000, there were 350,761 people, 125,807 households, and 86,303 families residing in the county. The population density was 1,552 people per square mile (599/km²). There were 133,280 housing units at an average density of 590 per square mile (228/km²). The racial makeup of the county was 68.48% White, 19.81% Black or African American, 0.20% Native American, 4.94% Asian, 0.10% Pacific Islander, 4.29% from other races, and 2.17% from two or more races. 9.66% of the population were Hispanic or Latino of any race. 15.5% were of Italian, 9.7% Irish, 8.2% German, 6.7% Polish and 5.7% English ancestry according to Census 2000.
There were 125,807 households out of which 32.80% had children under the age of 18 living with them, 50.60% were married couples living together, 13.80% had a female householder with no husband present, and 31.40% were non-families. 25.60% of all households were made up of individuals and 9.90% had someone living alone who was 65 years of age or older. The average household size was 2.62 and the average family size was 3.16.
In the county the population was spread out with 24.00% under the age of 18, 10.20% from 18 to 24, 30.60% from 25 to 44, 22.50% from 45 to 64, and 12.60% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 94.90 males. For every 100 females age 18 and over, there were 92.00 males.
The median income for a household in the county was $56,613, and the median income for a family was $68,494 (these figures had risen to $68,582 and $85,169 respectively as of a 2007 estimate). Males had a median income of $47,444 versus $34,788 for females. The per capita income for the county was $27,914. About 5.90% of families and 8.60% of the population were below the poverty line, including 10.60% of those under age 18 and 8.40% of those age 65 or over.
Mercer County has state routes, U.S. Routes and Interstates. Mercer also boasts its NJTransit's stations, including Trenton, Hamilton, and Princeton Junction. The state routes that pass through Mercer are Route 33, Route 29, Route 31, Route 27, Route 129, and Route 133. There are three US Routes that pass through Mercer, which are: U.S. Route 1, U.S. Route 130 and U.S. Route 206.
Mercer County houses a couple of limited access roads, such as Interstate 295, Interstate 195, and Interstate 95 (which is also designated along the New Jersey Turnpike). Two turnpike interchanges are located in Mercer, Exit 7A and Exit 8.
Interstate 95 abruptly ends at the interchange with US 1 & I-295 in Lawrence Township, and becomes I-295 south. Signs direct motorists to the continuation of I-95 by using I-295 to I-195 east to Interstate 95 / New Jersey Turnpike. This is all due in part to the cancellation of the Somerset Freeway that was supposed to go from Hopewell in Mercer County up to Franklin in Somerset County. However, the 95 shields on the "Trenton section" are to re-numbered as part of "I-195 Extension," when a direct interchange with Interstate 95 in Pennsylvania & Interstate 276 is built (which won't be until 2012).