Netcong, New Jersey. Netcong is a borough in Morris County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 3,232, reflecting an increase of 652 (+25.3%) from the 2,580 counted in the 2000 Census, which had in turn declined by 731 (-22.1%) from the 3,311 counted in the 1990 Census.