West Mifflin is a borough in Allegheny County, Pennsylvania, United States, located southeast of downtown Pittsburgh. The population was 20,313 at the 2010 census. It is named after Thomas Mifflin, 1st Governor of Pennsylvania, signer of the United States Constitution, and 1st Quartermaster General of the United States Army.
Although the borough is heavily residential, it is home to one of America's oldest traditional amusement parks, Kennywood Park. Other employers include advanced naval nuclear propulsion technology research and development facility, Bettis Atomic Power Laboratory; monorail manufacturer Bombardier; US Steel's Mon Valley Works–Irvin Plant; Community College of Allegheny County's South Campus; and the Allegheny County Airport.
According to the United States Census Bureau, the borough has a total area of 14.4 square miles (37 km2), of which 14.2 square miles (37 km2) is land and 0.3 square miles (0.78 km2), or 1.80%, is water. The landscape is largely hilly and wooded, and the borough's eastern boundary is contiguous with the Monongahela River three separate times. Much of the original landscape has been altered as a result of the historic dumping of steel mill byproducts such as slag and fly ash.
Coal mining has also affected the flow and water quality of small streams. Land developers have produced more level ground by clean-filling ravines and other small parcels of land to improve the land usage. Toxic waste dump areas are monitored with water quality improvement with bioremediation successfully implemented. West Mifflin operates its own sewage treatment facility.