Plainfield is a city in Union County, New Jersey, United States, known by its nickname as "The Queen City". As of the 2010 United States Census, the city's population increased to 49,808, its highest ever recorded population in any decennial census, with the population having increased by 1,979 from the 47,829 counted in the 2000 Census, which had in turn increased by 1,262 from the 46,567 counted in the 1990 Census.The area of present-day Plainfield was originally formed as Plainfield Township, a township that was created on April 5, 1847, from portions of Westfield Township, while the area was still part of Essex County. On March 19, 1857, Plainfield Township became part of the newly created Union County.Plainfield was incorporated as a city by an act of the New Jersey Legislature on April 21, 1869, from portions of Plainfield Township, based on the results of a referendum held that same day.