Mount Arlington is a borough in Morris County, New Jersey, United States. As of the 2000 United States Census, the borough's population was 5,050, reflecting an increase of 387 (+8.3%) from the 4,663 counted in the 2000 Census, which had in turn increased by 1,033 (+28.5%) from the 3,630 counted in the 1990 Census. It is located on the southeast shore of Lake Hopatcong, New Jersey's largest lake and a major recreational resource.
Mount Arlington was incorporated as a borough by an Act of the New Jersey Legislature on November 3, 1890, from portions of Hanover Township, based on the results of a referendum held two days earlier. Additional territory was acquired from Roxbury Township on July 25, 1891, on May 31, 1893, and on May 31, 1894.