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 (+6.5%) from the 2010 census count of 7,914, which in turn reflected an increase of 239 (+3.1%) from the 7,675 counted in the 2000 census.