Mahopac, NY. Online Offers. Mahopac, New York, is a hamlet (and census-designated place) in the Town of Carmel in Putnam County, New York. An exurb of New York City some 47 miles (76 km) to the south, Mahopac is located on US Route 6 on the County's southern central border with Westchester County. As of the 2000 census, the population was 8,478. Mahopac and Mahopac Falls play central roles in the history of Putnam County.