Plainfield was originally formed as a township on April 5, 1847, from portions of Westfield Township, while the area was still part of Essex County. On March 19, 1857, it became part of the newly-created Union County. Plainfield was incorporated as a city by an Act of the New Jersey Legislature on April 21, 1869, from portions of Plainfield Township, based on the results of a referendum held that same day. The city and township coexisted until March 6, 1878, when Plainfield Township was dissolved and parts absorbed by Plainfield City and the remainder becoming Fanwood Township (now known as Scotch Plains).
Plainfield is nicknamed "The Queen City".
As of the census of 2000, there were 47,829 people, 15,137 households, and 10,898 families residing in the city. The population density was 7,921.7 people per square mile (3,057.4/km²). There were 16,180 housing units at an average density of 2,679.8/sq mi (1,034.3/km²). The racial makeup of the city was 21.45% White, 61.78% African American, 0.41% Native American, 0.93% Asian, 0.10% Pacific Islander, 10.78% from other races, and 4.55% from two or more races. Hispanic or Latino of any race were 25.16% of the population.
There were 15,137 households out of which 35.5% had children under the age of 18 living with them, 39.3% were married couples living together, 24.5% had a female householder with no husband present, and 28.0% were non-families. 21.1% of all households were made up of individuals and 7.4% had someone living alone who was 65 years of age or older. The average household size was 3.10 and the average family size was 3.49.
In the city the population was spread out with 27.5% under the age of 18, 10.2% from 18 to 24, 32.6% from 25 to 44, 20.5% from 45 to 64, and 9.2% who were 65 years of age or older. The median age was 33 years. For every 100 females there were 95.7 males. For every 100 females age 18 and over, there were 92.2 males.
The median income for a household in the city was $46,683, and the median income for a family was $50,774. Males had a median income of $33,460 versus $30,408 for females. The per capita income for the city was $19,052. About 12.2% of families and 15.9% of the population were below the poverty line, including 21.3% of those under age 18 and 12.6% of those age 65 or over.
Plainfield has two New Jersey Transit rail stations on the Raritan Valley Line, formerly the mainline of the Central Railroad of New Jersey. The main Plainfield station is in the downtown and a second, smaller Netherwood station is in the Netherwood section, east of the downtown.
NJ Transit also provides bus service on the 113 and 114 to the Port Authority Bus Terminal in Midtown Manhattan; the 59, 65 and 66 (Limited) to Newark; and local service on the 822 and 819 routes.
Newark Liberty International Airport is approximately 25 minutes away.