Allegheny County, formed in 1788 from parts of Washington and Westmoreland Counties. County seat: Pittsburgh.
Armstrong County, formed in 1800 from parts of Allegheny, Lycoming, and Westmoreland Counties. County seat: Kittanning.
Beaver County, formed in 1800 from parts of Allegheny and Washington Counties. County seat: Beaver.
Bedford County, formed in 1771 from parts of Cumberland County. County seat: Bedford.
Berks County, formed in 1752 from parts of Chester, Lancaster and Philadelphia Counties. County seat: Reading.
Blair County, formed in 1846 from parts of Huntingdon and Bedford Counties. County seat: Hollidaysburg.
Bradford County, formed in 1810 from parts of Luzerne and Lycoming Counties; originally called Ontario County, renamed as Bradford County in 1812. County seat: Towanda.
Bucks County, one of the original counties at the formation of Pennsyvania in 1682. County seat: Doylestown.
Butler County, formed in 1800 from parts of Allegheny County. County seat: Butler.
Cambria County, formed in 1804 from parts of Somerset and Huntingdon Counties. County seat: Ebensburg.
Cameron County, formed in 1860 from parts of Clinton, Elk, McKean, and Potter Counties. County seat: Emporium.
Carbon County, formed in 1843 from parts of Monroe and Northampton Counties. County seat: Jim Thorpe.
Centre County, formed in 1800 from parts of Lycoming, Mifflin, Northumberland, and Huntingdon Counties. County seat: Bellefonte.
Chester County, one of the original counties at the formation of Pennsylvania in 1682. County seat: West Chester.
Clarion County, formed in 1839 from parts of Venango and Armstrong Counties. County seat: Clarion.
Clearfield County, formed in 1804 from parts of Lycoming and Huntingdon Counties; Clearfield functioned as a part of Centre County for judiciary purposes until 1822. County seat: Clearfield.
Clinton County, formed in 1839 from parts of Lycoming and Centre Counties. County seat: Lock Haven.
Columbia County, formed in 1813 from parts of Northumberland and Luzerne Counties. County seat: Bloomsburg.
Indiana County, formed in 1803 from parts of Lycoming and Westmoreland Counties; it was attached to Westmoreland County until 1806. County seat: Indiana.
Jefferson County, formed in 1804 from parts of Lycoming County. Attached to Westmoreland County until 1806 and to Indiana County until 1830. County seat: Brookville.
McKean County, formed in 1804 from parts of Lycoming County; Attached to Centre County until 1814 and to Lycoming County until 1826 for judicial and elective purposes. McKean was fully organized only in 1826. County seat: Smethport.
Mercer County, formed in 1800 from parts of Allegheny County. County seat: Mercer.
Mifflin County, formed in 1789 from parts of Cumberland and Northumberland Counties. County seat: Lewistown.
Monroe County, formed in 1836 from parts of Pike and Northampton Counties. County seat: Stroudsburg.
Northumberland County, formed in 1772 from parts of Lancaster, Berks, Bedford, Cumberland, and Northampton Counties. County seat: Sunbury.
Perry County, formed in 1820 from parts of Cumberland County. County seat: New Bloomfield.
Philadelphia County, one of the original counties at the formation of Pennsylvania in 1682. The city and county of Philadelphia were combined in 1854 and city and county offices merged in 1952.
Pike County, formed in 1814 from parts of Wayne County. County seat: Milford.
Potter County, Formed in 1804 from Lycoming county. Attached to Lycoming County until 18265 and to McKean County until 1835 for judicial purposes, Potter was not fully organized until 1835. County seat: Coudersport.
Sullivan County, formed in 1847 from parts of Lycoming County; attached to Lycoming until 1848. County seat: Laporte.
Susquehanna County, formed in 1810 from parts of Luzerne County ; attached to Luzerne County until 1812. County seat: Montrose.
Tioga County, formed in 1804 from parts of Lycoming County; attached to Lycoming until 1812. County seat: Wellsboro.
Union County, formed in 1813 from parts of Northumberland County. County seat: Lewisburg.
Venango County, formed in 1800 from parts of Allegheny and Lycoming Counties; attached to until 1805. County seat: Franklin.
Warren County, formed in 1800 from parts of Allegheny and Lycoming counties; attached to Crawford County until 1805 and then to Venango until Warren was formally organized in 1819. County seat: Warren.