South Plainfield was incorporated by an Act of the New Jersey Legislature on March 10, 1926, from portions of Piscataway Township, based on the results of a referendum passed on April 6, 1926.
As of the census of 2000, there were 21,810 people, 7,151 households, and 5,856 families residing in the borough. The population density was 2,609.8 people per square mile (1,007.3/km²). There were 7,307 housing units at an average density of 874.3/sq mi (337.5/km²). The racial makeup of the borough was 77.74% White, 8.56% African American, 0.22% Native American, 7.57% Asian, 3.48% from other races, and 2.42% from two or more races. Hispanic or Latino of any race were 8.66% of the population.
There were 7,151 households out of which 37.6% had children under the age of 18 living with them, 66.8% were married couples living together, 10.9% had a female householder with no husband present, and 18.1% were non-families. 15.3% of all households were made up of individuals and 7.5% had someone living alone who was 65 years of age or older. The average household size was 3.01 and the average family size was 3.35.
In the borough the population was spread out with 25.1% under the age of 18, 7.2% from 18 to 24, 29.9% from 25 to 44, 23.7% from 45 to 64, and 14.1% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 96.1 males. For every 100 females age 18 and over, there were 92.6 males.
The median income for a household in the borough was $67,466, and the median income for a family was $72,745 (these figures had risen to $88,425 and $92,817 respectively as of a 2007 estimate. Males had a median income of $47,465 versus $34,329 for females. The per capita income for the borough was $25,270. About 2.3% of families and 3.4% of the population were below the poverty line, including 3.7% of those under age 18 and 4.4% of those age 65 or over.