New Castle is a city in and the county seat of Lawrence County, Pennsylvania, United States, 50 miles northwest of Pittsburgh and near the Pennsylvania–Ohio border just 18 miles east of Youngstown, Ohio. The population was 23,128 as of the 2010 census. It is the commercial center of a fertile agricultural region.New Castle is the principal city of the New Castle, PA Micropolitan Statistical Area, which had a population of 91,108 in 2010. New Castle also anchors the northwestern part of the Pittsburgh-New Castle-Weirton, PA-WV-OH Combined Statistical Area.