High Quality Content by WIKIPEDIA articles! 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. The population increased by 833 (+3.9%) from the 21,103 counted in the 2000 Census, which had in turn increased by 2,283 (+12.1%) from the 18,820 counted in the 1990 Census.