Mercer County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 110,652. Its county seat is Mercer, and its largest city is Hermitage. The county is part of the Northwest Pennsylvania region of the state.

Hermitage is a city in western Mercer County, Pennsylvania, United States. The population was 16,230 at the 2020 census, making it the largest city in Mercer County. It is located about 15 miles (24 km) northeast of Youngstown and about 60 miles (97 km) northwest of Pittsburgh. It is the principal city of the Hermitage micropolitan area, which includes all of Mercer County.