Kenilworth is a borough in Union County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 7,914, reflecting an increase of 239 from the 7,675 counted in the 2000 Census, which had in turn increased by 101 from the 7,574 counted in the 1990 Census.Kenilworth was incorporated as a borough by an act of the New Jersey Legislature on May 13, 1907, from portions of Cranford and Union Township, based on the results of a referendum held on June 18, 1907.