West Mifflin is a borough in Allegheny County, Pennsylvania, United States, located southeast of downtown Pittsburgh. The population was 22,464 at the 2000 census. Although the borough is heavily residential, it is home to one of America's oldest traditional amusement parks, Kennywood Park; the fourth largest shopping mall in the region, Century III; a nuclear research and development facility, Bettis Atomic Power Laboratory; monorail manufacturer Bombardier; P.J. Dick Trumbull-Corporation; US Steel's Irvin Plant; Community College of Allegheny County's South Campus; the Allegheny County Airport, and various other businesses and industries.