Adams, created 1800 from York.
Allegheny, created 1788 from Westmoreland, Washington.
Armstrong, created 1800 from Allegheny, Lycoming, Westmoreland.
Beaver, created 1800 from Allegheny, Washington
Bedford, created 1710 from Cumberland.
Berks, created 1752 from Chester, Lancaster, Philadelphia.
Blair, created 1846 from Huntington, Bedford.
Bradford, created 1810 from Luzerne, Lycoming (name changed from Ontario in 1812).
Bucks, created 1682, original county.
Butler, created 1800 from Allegheny.
Cambria, created 1804 from Somerset, Bedford, Huntingdon.
Cameron, created 1860 from Clinton, Elk, McKean, Potter.
Carbon, created 1843 from Northampton, Monroe.
Centre, created 1800 from Lycoming, Mifflin, Northumberland, Huntingdon.
Chester, created 1682, original county.
Clarion, created 1839 from Venango, Armstrong.
Clearfield, created 1804 from Huntingdon, Lycoming.
Clinton, created 1839 from Lycoming, Centre.
Columbia, created 1813 from Northumberland.
Crawford, created 1800 from Allegheny.
Cumberland, created 1750 from Lancaster.
Dauphin, created 1785 from Lancaster.
Delaware, created 1789 from Chester.
Elk, created 1843 from Jefferson, McKean, Clearfield.
Erie, created 1800 from Allegheny.
Fayette, created 1783 from Westmoreland.
Forest, created 1848 from Jefferson, and 1866 from Venango.
Franklin, created 1784 from Cumberland.
Fulton, created 1850 from Bedford.
Greene, created 1796 from Washington.
Huntingdon, created 1787 from Bedford.
Indiana, created 1803 from Westmoreland, Lycoming.
Jefferson, created 1804 from Lycoming.
Juniata, created 1831 from Mifflin.
Lackawanna, created 1878 from Luzerne.
Lancaster, created 1729 from Chester.
Lawrence, created 1849 from Beaver, Mercer.
Lebanon, created 1813 from Dauphin, Lancaster.
Lehigh, created 1812 from Northampton.
Luzerne, created 1786 from Northumberland.
Lycoming, created 1795 from Northumberland.
McKean, created 1804 from Lycoming.
Mercer, created 1800 from Allegheny.
Mifflin, created 1789 from Cumberland, Northumberland.
Monroe, created 1836 from Pike, Northampton.
Montgomery, created 1784 from Philadelphia.
Montour, created 1850 from Columbia.
Northampton, created 1752 from Bucks.
Northumberland, created 1772 from Lancaster, Bedford, Berks, Northampton, Cumberland.
Ontario (see Bradford).
Perry, created 1820 from Cumberland.
Philadelphia, created 1682, original county.
Pike, created 1814 from Wayne.
Potter, created 1804 from Lycoming.
Schuylkill, created 1811 from Berks, Northampton and 1818 from Columbia and Luzerne.
Snyder, created 1855 from Union.
Somerset, created 1795 from Bedford.
Sullivan, created 1847 from Lycoming.
Susquehanna, created 1810 from Luzerne.
Tioga, created 1804 from Lycoming.
Union, created 1813 from Northumberland.
Venango, created 1800 from Allegheny, Lycoming.
Warren, created 1800 from Allegheny, Lycoming.
Washington, created 1781 from Westmoreland.
Wayne, created 1798 from Northampton.
Westmoreland, created 1773 from Bedford.
Wyoming, created 1842 from Luzerne.
York, created 1749 from Lancaster.