Monmouth, IL -- U.S. city in Illinois Population (2000): 9841 Housing Units (2000): 3986 Land area (2000): 4.029440 sq. miles (10.436202 sq. km) Water area (2000): 0.021109 sq. miles (0.054671 sq. km) Total area (2000): 4.050549 sq. miles (10.490873 sq. km) FIPS code: 50010 Located within: Illinois (IL), FIPS 17 Location: 40.911531 N, 90.644579 W ZIP Codes (1990): 61462 Note: some ZIP codes may be omitted esp. for suburbs. Headwords: Monmouth, IL Monmouth