New Castle is a city in Lawrence County, Pennsylvania, United States, northwest of Pittsburgh and near the Pennsylvania-Ohio border just east of Youngstown, Ohio; in 1910, the total population was 36,280; in 1920, 44,938; and in 1940, 47,638. The population has fallen to 23,128 according to the 2010 census. It is the county seat of Lawrence County. New Castle is the principal city of the New Castle micropolitan area and a part of the Pittsburgh–New Castle Combined Statistical Area, which is the 18th largest in the United States. It is the commercial center of a fertile agricultural region.