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North Plainfield is a Borough in Somerset County, New Jersey, United States. As of the 2010 United States Census, the borough population was 21,936. North Plainfield Township was created from portions of Warren Township, New Jersey by an Act of the New Jersey Legislature passed on April 2, 1872. The borough of North Plainfield became an independent municipality on June 9, 1885. North Plainfield is located at 40Â°37â²22â³N 74Â°26â²09â³W / 40.622735Â°N 74.435784Â°W / 40.622735; -74.435784 (40.622735, -74.435784).