what county is summit nj?
Summit, New Jersey. Summit is a city in Union County, New Jersey, United States. At the 2010 United States Census, the city's population was 21,457, reflecting an increase of 326 (+1.5%) from the 21,131 counted in the 2000 Census, which had in turn increased by 1,374 (+7.0%) from the 19,757 counted in the 1990 Census.