New Castle is a city in Lawrence County, Pennsylvania, United States, 50 miles (80 km) northwest of Pittsburgh and near the Pennsylvania-Ohio border just 18 miles (30 km) 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 26,309 according to the 2000 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. New Castle is a prime example of a Rust Belt city which has been forced to adapt to changing economic situations.