Mercer is a borough in and the county seat of Mercer County, Pennsylvania, United States. The population was 1,982 at the 2020 census. It is part of the Hermitage micropolitan area. The community was named after Brigadier General Hugh Mercer. The Mercer County Court House and Christiana Lindsey House are listed on the National Regis…Mercer is a borough in and the county seat of Mercer County, Pennsylvania, United States. The population was 1,982 at the 2020 census. It is part of the Hermitage micropolitan area. The community was named after Brigadier General Hugh Mercer. The Mercer County Court House and Christiana Lindsey House are listed on the National Register of Historic Places.