The borough is situated on the border with Union County in the Raritan Valley region, within the New York metropolitan area.

[21] South Plainfield was incorporated by an act of the New Jersey Legislature on March 12, 1926, from portions of Piscataway, based on the results of a referendum passed on April 6, 1926.

[26][27][28] The 2010 United States census counted 23,385 people, 7,876 households, and 6,175 families in the borough.

[33] As of the 2000 United States census[16] there were 21,810 people, 7,151 households, and 5,856 families residing in the borough.

[31][32] Tumi Inc. is a manufacturer of suitcases and bags for travel that was founded in 1975 by Charlie Clifford after serving in Peru with the Peace Corps.

[34] PTC Therapeutics is a pharmaceutical company focused on the development of small molecule, orally administered treatments for orphan diseases.

[6] The borough form of government used by South Plainfield is a "weak mayor / strong council" government in which council members act as the legislative body with the mayor presiding at meetings and voting only in the event of a tie.

The mayor can veto ordinances subject to an override by a two-thirds majority vote of the council.

[39][40] As of 2024[update], the mayor of South Plainfield is Republican Matthew P. Anesh, whose term of office ends December 31, 2026.

[3] Members of the South Plainfield Borough Council are Council President Robert A. Bengivenga Jr. (R, 2024), Christine Noonan Faustini (R, 2026), Melanie McCann-Mott (R, 2025), Peter D. Smith (R, 2025), Derryck C. White (R, 2026) and Joseph Wolak (R, 2024).

[52] For the 2024-2025 session, the 18th legislative district of the New Jersey Legislature is represented in the State Senate by Patrick J. Diegnan (D, South Plainfield) and in the General Assembly by Robert Karabinchak (D, Edison) and Sterley Stanley (D, East Brunswick).

[54] As of 2025[update], Middlesex County's Commissioners (with party affiliation, term-end year, and residence listed in parentheses) are: Director Ronald G. Rios (D, Carteret, 2027),[55] Deputy Director Shanti Narra (D, North Brunswick, 2027),[56] Claribel A.

[83] The South Plainfield Public Schools serve students in pre-kindergarten through twelfth grade.

[85] The schools in the district (with 2020–21 enrollment data from the National Center for Education Statistics).

[98] Al-Minhaal Academy is a private Islamic school serving students in kindergarten through twelfth grade.

[99] Lincoln Tech's campus in South Plainfield (formerly known as Engine City Technical Institute) is an accredited, diesel technology school that was established in 1969 as a training center to accommodate the need for diesel mechanics.

[101] Interstate 287 passes through the southern area of the borough for almost 2 miles (3.2 km), including exits 4 and 5.