Mahwah, New Jersey. For the river named Mahwah, see Mahwah River. Mahwah is a township in Bergen County, New Jersey, United States. As of the 2010 United States Census, the township's population was 25,890. The population increased by 1,828 (+7.6%) from the 24,062 counted in the 2000 Census, which had in turn increased by 6,157 (+34.4%) from the 17,905 counted in the 1990 Census.