Lyndhurst, New Jersey. Lyndhurst is a township in Bergen County, New Jersey, United States. As of the 2010 United States Census, the township's population was 20,554, reflecting an increase of 1,171 (+6.0%) from the 19,383 counted in the 2000 Census, which had in turn increased by 1,121 (+6.1%) from the 18,262 counted in the 1990 Census.