This is from Treasures of the Past
Origins of Pennsylvania Counties
Maps of how the counties in PA were formed
| 1728 | 1729 | 1770 | 1771 |
| 1772 | 1773 | 1785 | 1790 |
| 1800 | 1810 | 1820 | 1840 |
| 1850 | 1878 | Today | . |
The blue line through certain counties denotes an original county
| County | From | Year Formed |
| Adams | York | 1800 |
| Allegheny | Westmoreland, Washington | 1780 |
| Armstrong | Allegheny, Lycoming, and Westmoreland | 1800 |
| Beaver | Allegheny, Washington | 1800 |
| Bedford | Cumberland | 1771 |
| Berks | Philadelphia, Chester and Lancaster | 1752 |
| Blair | Huntingdon, Bedford | 1846 |
| Bradford | Luzerne, Lycoming | 1810 |
| Bucks | 1682 | |
| Butler | Allegheny | 1800 |
| Cambria | Huntingdon, Bedford and Somerset | 1804 |
| Cameron | Clinton, Elk, McKean and Potter | 1860 |
| Carbon | Northampton and Monroe | 1843 |
| Centre | Mifflin, Northumberland, Lycoming and Huntingdon | 1800 |
| Chester | 1682 | |
| Clarion | Venango and Armstrong | 1839 |
| Clearfield | Huntingdon and Lycoming | 1804 |
| Clinton | Lycoming and Centre | 1839 |
| Columbia | Northumberland | 1813 |
| Crawford | Allegheny | 1800 |
| Cumberland | Lancaster | 1750 |
| Dauphin | Lancaster | 1785 |
| Delaware | Chester | 1789 |
| Elk | Jefferson, Clearfield and McKean | 1843 |
| Erie | Allegheny | 1800 |
| Fayette | Westmoreland | 1783 |
| Forest | Jefferson and Venango | 1848 |
| Franklin | Cumberland | 1748 |
| Fulton | Bedford | 1850 |
| Greene | Washington | 1786 |
| Huntingdon | Bedford | 1787 |
| Indiana | Westmoreland and Lycoming | 1803 |
| Jefferson | Lycoming | 1804 |
| Juniata | Mifflin | 1831 |
| Lackawanna | Luzerne | 1878 |
| Lancaster | Chester | 1729 |
| Lawrence | Beaver and Mercer | 1849 |
| Lebanon | Dauphin and Lancaster | 1813 |
| Lehigh | Northampton | 1812 |
| Luzerne | Northumberland | 1786 |
| McKean | Lycoming | 1804 |
| Mercer | Allegheny | 1800 |
| Mifflin | Cumberland and Northumberland | 1789 |
| Monroe | Northampton | 1836 |
| Montgomery | Philadelphia | 1784 |
| Montour | Columbia | 1850 |
| Northampton | Bucks | 1752 |
| Northumberland | Lancaster, Berks, Northampton, Cumberland and Bedford | 1772 |
| Perry | Cumberland | 1820 |
| Philadelphia | 1682 | |
| Pike | Wayne | 1814 |
| Potter | Lycoming | 1804 |
| Schuylkill | Berks and Northampton | 1811 |
| Snyder | Union | 1855 |
| Somerset | Bedford | 1795 |
| Sullivan | Lycoming and Bradford | 1847 |
| Susquehanna | Luzerne | 1810 |
| Tioga | Lycoming | 1804 |
| Union | Northumberland | 1813 |
| Venango | Allegheny and Lycoming | 1800 |
| Warren | Allegheny and Lycoming | 1800 |
| Washington | Westmoreland | 1781 |
| Wayne | Northampton | 1798 |
| Westmoreland | Bedford, Purchase of 1784 | 1773 |
| Wyoming | Luzerne | 1842 |
| York | Lancaster | 1749 |
