Millvale is a borough in Allegheny County, Pennsylvania, United States, along the Allegheny River, opposite Pittsburgh. The borough is located off Pennsylvania Route 28. The population was 3,376 …Millvale is a borough in Allegheny County, Pennsylvania, United States, along the Allegheny River, opposite Pittsburgh. The borough is located off Pennsylvania Route 28. The population was 3,376 at the 2020 census.