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 (−2.3%) 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.

Morris County is a county located in the U.S. state of New Jersey, about 30 mi (48 km) west of New York City. According to the 2020 census, the county was the state's tenth-most populous county, with a population of 509,285, its highest decennial count ever and an increase of 17,009 (+3.5%) from the 2010 census count of 492,276, which in turn reflected an increase of 22,064 (+11.6%) from the 470,212 counted in the 2000 census, Morris County is part of the New York metropolitan area and is divided into 39 municipalities, with many commuter towns but no large cities. Its county seat is Morristown, in the southeast. The most populous place was Parsippany-Troy Hills Township, with 56,162 residents at the time of the 2020 census, while Rockaway Township covered 45.55 square miles (118.0 km2), the largest total area of any municipality. The county is part of the North Jersey region of the state.