Mount Arlington is a borough in western 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 from the 2010 census count of 5,050, which in turn reflected an increase of 387 from the 4,663 counted in the 2000 census. The borough is located on the southeast shore of Lake Hopatcong, New Jersey's largest lake and a major recreational resource. Mount Arlington is home to the Lake Hopatcong Yacht Club.