As of the census of 2000, there were 522,541 people, 186,124 households, and 133,264 families residing in the county. The population density was 5,059 people per square mile (1,953/km²). There were 192,945 housing units at an average density of 1,868 per square mile (721/km²). The racial makeup of the county was 65.51% White, 20.78% Black or African American, 0.23% Native American, 3.83% Asian, 0.04% Pacific Islander, 6.37% from other races, and 3.25% from two or more races. 19.71% of the population were Hispanic or Latino of any race. 13.1% were of Italian, 8.6% Irish, 6.5% Polish and 5.8% German ancestry according to Census 2000.
There were 186,124 households out of which 34.00% had children under the age of 18 living with them, 52.60% were married couples living together, 14.20% had a female householder with no husband present, and 28.40% were non-families. 23.60% of all households were made up of individuals and 10.20% had someone living alone who was 65 years of age or older. The average household size was 2.77 and the average family size was 3.28.
In the county the population was spread out with 24.90% under the age of 18, 7.90% from 18 to 24, 31.30% from 25 to 44, 22.10% from 45 to 64, and 13.80% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 92.70 males. For every 100 females age 18 and over, there were 88.90 males.
The median income for a household in the county was $55,339, and the median income for a family was $65,234. Males had a median income of $44,544 versus $32,487 for females. The per capita income for the county was $26,992. About 6.30% of families and 8.40% of the population were below the poverty line, including 10.50% of those under age 18 and 8.00% of those age 65 or over.
Union County is extremely diverse. Berkeley Heights, New Providence, Westfield, Summit, Cranford, Kenilworth, Clark, Linden, Union, Springfield and Scotch Plains have a very high number of Italian American residents as well as a large number of Irish Americans and residents of Northern European descent in general. Plainfield, Roselle, Linden, Union, Rahway, and Elizabeth all have large African American communities. Roselle Park has a notably large Indian American community, and Roselle Park, Linden, Rahway, and Plainfield, and particularly Elizabeth, have fast-growing Hispanic and Portuguese populations. There are fair-sized Jewish-American communities in Springfield, Scotch Plains, Elizabeth, Cranford, Westfield, and Summit, though not on the level of Essex County.
The county is served by numerous transportation modes including rail, air, highways and ports.
Major highways which traverse the county include the New Jersey Turnpike, Garden State Parkway, Interstate 78, Interstate 278, U.S. Route 1, U.S. Route 9, U.S. Route 22 and the Goethals Bridge.
Passenger rail service is provide by New Jersey Transit via the Northeast Corridor, North Jersey Coast Line, Raritan Valley Line, the Morristown Line and the Gladstone Branch. Freight service is provided by on Conrail's Lehigh Line and Chemical Coast Branch.
The Elizabeth Marine Terminal is part of the Port Authority of New York and New Jersey.
The southern portion of Newark Liberty International Airport is located in Elizabeth, within Union County.