Kenilworth is a borough in Union County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 8,427, an increase of 513 from the 2010 census count of 7,914, which in turn reflected an increase of 239 from the 7,675 counted in the 2000 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.