Summit is the northernmost city of Union County, in the U.S. state of New Jersey, located within the New York metropolitan area. Situated on a …Summit is the northernmost city of Union County, in the U.S. state of New Jersey, located within the New York metropolitan area. Situated on a ridge in northern-central New Jersey, the city is located within the Raritan Valley and Rahway Valley regions, and also borders both Essex and Morris counties in the Passaic Valley region. Summit is a commercial hub and commuter town for New York City. As of the 2020 United States census, the city's population was 22,719, an increase of 1,262 from the 2010 census count of 21,457, which in turn reflected an increase of 326 from the 21,131 counted in the 2000 census.