in what county mendham nj  in
Mendham Borough 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.