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.