Mount Arlington is a borough in Morris County, New Jersey, United States. As of the United States 2000 Census, the borough population was 4,663. It is located on the southeast shore of Lake Hopatcong, New Jersey's largest lake (man made) 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.