Warren County was incorporated by an Act of the New Jersey Legislature on November 20, 1824, from portions of Sussex County. At its creation, the county consisted of the townships of Greenwich, Independence, Knowlton, Mansfield, Oxford, Pahaquarry (now defunct).
Warren County is generally considered the eastern border of the Lehigh Valley.
As of the census of 2000, there were 102,437 people, 38,660 households, and 27,487 families residing in the county. The population density was 286 people per square mile (111/km²). There were 41,157 housing units at an average density of 115 per square mile (44/km²). The racial makeup of the county was 94.54% White, 1.87% Black or African American, 0.11% Native American, 1.21% Asian, 0.02% Pacific Islander, 1.01% from other races, and 1.24% from two or more races. 3.66% of the population were Hispanic or Latino of any race. 18.4% were of Italian, 18.2% German, 14.6% Irish, 7.2% Polish, 6.8% English and 5.2% American ancestry according to Census 2000.
There were 38,660 households out of which 34.70% had children under the age of 18 living with them, 58.20% were married couples living together, 9.20% had a female householder with no husband present, and 28.90% were non-families. 24.00% of all households were made up of individuals and 10.00% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.12.
In the county the population was spread out with 26.10% under the age of 18, 6.30% from 18 to 24, 31.30% from 25 to 44, 23.50% from 45 to 64, and 12.90% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 94.90 males. For every 100 females age 18 and over, there were 91.10 males.
The median income for a household in the county was $56,100, and the median income for a family was $66,223. Males had a median income of $47,331 versus $31,790 for females. The per capita income for the county was $25,728. About 3.60% of families and 5.40% of the population were below the poverty line, including 5.90% of those under age 18 and 6.70% of those age 65 or over.
While Warren County only has one New Jersey Transit train stop in the entire county (Hackettstown), Warren has a number of state routes, a few U.S. Routes, and two interstates. Warren houses Route 57, Route 173, Route 31, and Route 94. The US Routes are U.S. Route 22, and U.S. Route 46. The two interstates that pass through the county are the Phillipsburg-Newark Expressway (I-78), and the Bergen-Passaic Expressway (I-80).