Mendham Borough 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 4,981, unchanged from the 2010 census, which in turn reflected a decline of 116 from the 5,097 counted in the 2000 census. Located in the Raritan Valley region within the New York Metropolitan area, the North Branch of the Raritan River begins in Mendham Borough and flows in a southwest direction towards neighboring Somerset County.