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.
The borough is known for the Mendham Historic District, which is listed on the National Register of Historic Places with notable landmarks the Phoenix House, which serves as the Borough's Municipal Building, and the historic Black Horse Inn and Tavern. New Jersey Monthly magazine ranked Mendham Borough as the best place to live in the state in its 2013 rankings of the "Best Places to Live" in New Jersey. In the Forbes magazine's 2006 (209th; median sale price of $835,000) and 2012 (356th; $800,672) rankings of the Most Expensive ZIP Codes in the United States, Mendham was listed among the top 500 nationwide.