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 (29 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, and continues to decline with a Census Bureau estimate of 24,060 for 2009. 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.