Mendham is a borough in Morris County, New Jersey, in the United States. As of the 2010 United States Census, the borough's population was 4,981, reflecting a decline of 116 (-2.3%) from the 5,097 counted in the 2000 Census, which had in turn increased by 207 (+4.2%) from the 4,890 counted in the 1990 Census.
Mendham Township, which included present-day Mendham Borough, was formed on March 29, 1749, from portions of Hanover Township, Morris Township and Rockaway Township. The township was re-incorporated under the same name by an Act of the New Jersey Legislature on February 21, 1798. Mendham was incorporated as a borough by an Act of the New Jersey Legislature on May 15, 1906, from portions of Mendham Township. Residents of what became Mendham Borough wanted sidewalks and street lights constructed while those that lived in what remained the Township (including the communities of Brookside and Ralston) felt it was cost prohibitive in their more rural areas.