[25] In 1902, the New Jersey Legislature approved measures that would have allowed the borough to become part of Union County (a measure repealed in 1903) and to allow for a merger of North Plainfield with the City of Plainfield subject to the approval of a referendum by voters in both municipalities.

[29][30][31] North Plainfield is in the northern division of the Raritan Valley along with Green Brook.

The 2010 United States census counted 21,936 people, 7,448 households, and 5,266 families in the borough.

[38] As of the 2000 United States census[16] there were 21,103 people, 7,202 households, and 5,084 families residing in the borough.

[36][37] As of the 2000 Census, 5.39% of North Plainfield's residents identified themselves as being of Ecuadorian ancestry, which was the fourth highest of any municipality in New Jersey and the eighth highest percentage of Ecuadorian people in any place in the United States with 1,000 or more residents identifying their ancestry.

[39] The Borough of North Plainfield is governed within the Faulkner Act under the Mayor-Council system of municipal government (Plan E), implemented as of January 1, 1977, based on the recommendations of a Charter Study Commission.

This form provides for a strong-mayor type of government, in which the mayor has executive functions and the legislative branch is the borough council.

[6][42] The Municipal Judge is appointed by the mayor with the advice and consent of the council, and serves for three years.

The mayor runs the borough on a day-to-day basis and ensures the enforcement of the legislation passed by the council.

"Skip" Stabile III (D, 2024), Council Vice President Everett Merrill (D, 2022), Aimee Corzo (D, 2024), Suezette Given (D, 2022; elected to serve an unexpired term), Steve McIntyre (D, 2022; elected to serve an unexpired term), Keiona R. Miller (D, 2024) and Wendy Schaefer (D, 2022).

[49] Later that month, the borough council selected Suezette Given from a list of three submitted individuals to fill the seat expiring in December 2022 that had been held by Douglass M. Singleterry until he resigned from office on the last day of December to take office on the Somerset County Board of County Commissioners.

[46] In August 2018, the borough council selected Aimee Corzo from a list of three candidates nominated by the Democratic municipal committee to fill the seat expiring in December 2020 that had been held by Frank Righetti until he resigned from office.

[53][54][55] For the 119th United States Congress, New Jersey's 12th congressional district is represented by Bonnie Watson Coleman (D, Ewing Township).

[58] For the 2024-2025 session, the 22nd legislative district of the New Jersey Legislature is represented in the State Senate by Nicholas Scutari (D, Linden) and in the General Assembly by Linda S. Carter (D, Plainfield) and James J. Kennedy (D, Rahway).

[59] Somerset County is governed by a five-member Board of County Commissioners, whose members are elected at-large to three-year terms of office on a staggered basis, with one or two seats coming up for election each year.

[76] Among the borough's 2010 Census population, 44.4% (vs. 60.4% in Somerset County) were registered to vote, including 58.8% of those ages 18 and over (vs. 80.4% countywide).

[82][83] In the 2009 gubernatorial election, Democrat Jon Corzine received 2,119 ballots cast (50.3% vs. 34.1% countywide), ahead of Republican Chris Christie with 1,744 votes (41.4% vs. 55.8%), Independent Chris Daggett with 255 votes (6.1% vs. 8.7%) and other candidates with 46 votes (1.1% vs. 0.7%), among the 4,214 ballots cast by the borough's 9,840 registered voters, yielding a 42.8% turnout (vs. 52.5% in the county).