Mountainside is a borough in Union County, in the U.S. state of New Jersey. The borough is located on a ridge in northern-central New Jersey, within the Raritan Valley and Rahway Valley regions in the New York metropolitan area. As of the 2020 United States census, the borough's population was 7,020, an increase of 335 from the 2010 census count of 6,685, which in turn reflected an increase of 83 from the 6,602 counted in the 2000 census.