Plainfield, NJ - Plainfield, New Jersey Map & Directions
Plainfield Map. Plainfield is a city in Union County, New Jersey, United States.As of the 2010 United States Census, the city's population increased to a record high of 49,808. Plainfield was originally formed as a township on April 5, 1847, from portions of Westfield Township, while the area was still part of Essex County.