Mount Arlington is a borough in southwestern Morris County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 5,909, an increase of 859 (+17.0%) from the 2010 census count of 5,050, which in turn reflected an increase of 387 (+8.3%) from the 4,663 counted in the 2000 census.

Lake Hopatcong is the largest freshwater body in New Jersey, United States, about 4 square miles (10 km2) in area. Located 30 miles (48 km) from the Delaware River and 40 miles (64 km) from Manhattan, New York City, the lake forms part of the border between Sussex and Morris counties in the state's northern highlands region.