Mountainside, New Jersey, United States [Borough]; population was 6,657 in 1990; housing units was 2,454 in 1990; location is 40°41'N 74°22'W; land area is 4.02 square miles (2,574 acres); water area is 0.03 square miles (21 acres); FIPS code is 48510 [SourceCBP]