Pennsylvania County Creation Dates and Parent Counties

Name of County	Date Formed	Boundary Changes	Parent County	Counties Created by Boundary Changes
Adams	1800 Jan 22	Details	York	none
Allegheny	1788 Sep 24	Details	Washington, Westmoreland	Armstrong, Beaver, Venango, Warren, Butler, Crawford, Erie, and Mercer 1800
Armstrong	1800 Mar 12	Details	Allegheny, Westmoreland, Lycoming. Attached to Westmoreland County until 1805.	Clarion 1839
Beaver	1800 Mar 12	Details	Allegheny, Washington. Attached to Allegheny Co. until 1803
Bedford	1771 Mar 09	Details	Cumberland	Northumberland 1772, Westmoreland 1773, Huntingdon 1787, Somerset 1795, Cambria 1804, Blair 1846, Fulton 1850
Berks	1752 Oct 14	Details	Philadelphia, Chester, Lancaster	Northumberland 1772, Schuylkill 1811
Blair	1846 Feb 26	Details	Huntingdon, Bedford	none
Bradford	1812 Feb 21	Details	renamed fromm Ontario 1812	none
Bucks	1682 Mar 10	Details	none	Northampton1751
Butler	1800 Mar 12	Details	Allegheny	none
Cambria	1807 Mar 26	Details	Huntingdon, Somerset, and Bedford	none
Cameron	1860 Mar 29	Details	Clinton, McKean, Elk, Potter	none
Carbon	1843 Mar 13	Details	Northampton, Monroe	none
Centre	1800 Feb 13	Details	Huntingdon, Lycoming, Mifflin, Northumberland	Clinton 1839
Chester	1682 Mar 10	Details	Colonial Lands	Lancaster 1728, Berks 1751, Delaware in 1789
Clarion	1839 Mar 11	Details	Venango, Armstrong	none
Clearfield	1804 Mar 26	Details	Huntingdon and Lycoming (Clearfield did not have its own courts until 1822.)	Elk 1843
Clinton	1839 Jan 21	Details	Centre and Lycoming	Cameron 1860
Columbia	1813 Mar 22	Details	Northumberland	Additional land went to Schuylkill 1818, Montour 1850
Crawford	1800 Mar 12	Details	Allegheny	No counties were formed from Crawford Co. Venango, Erie, Warren, and Mercer counties were attached to Crawford Co. for several years.
Cumberland	1750 Jan 27	Details	Lancaster	Bedford 1771, Northumberland 1772, Franklin 1784, Mifflin 1789, Perry 1820
Dauphin	1785 Mar 04	Details	Lancaster	Lebanon 1813
Delaware	1789 Sep 26	Details	Chester	none
Elk	1843 Apr 18	Details	Jefferson, Clearfield, McKean	Cameron 1860
Erie	1800 Mar 12	Details	Allegheny (Attached to Crawford Co. for several years.)	none
Fayette	1783 Sep 26	Details	Westmoreland	none
Forest	1848 Apr 11	Details	Jefferson	none
Franklin	1784 Sep 09	Details	Cumberland	none
Fulton	1850 Apr 19	Details	Bedford	none
Greene	1796 Feb 09	Details	Washington	none
Huntingdon	1787 Sep 20	Details	Bedford	Centre 1800, Cambria and Clearfield 1804, Blair 1846
Indiana	1803 Mar 30	Details	Westmoreland, Lycoming (attached to Westmoreland Co. until 1806 when county was organized.)	none
Jefferson	1804 Mar 26	Details	Lycoming (Attached to Westmoreland Co. until 1806, then to Indiana Co. until 1830.)	Elk 1843, Forest 1848
Juniata	1831 Mar 02	Details	Mifflin	none
Lackawanna	1878 Aug 21	Details	Luzerne	none
Lancaster	1728 Oct 14	Details	Chester (Earlier was part of Susquehannock Indian territory.)	York 1748, Cumberland 1750, Berks 1751, Northumberland 1772, Dauphin 1785, Lebanon 1813
Lawrence	1849 Mar 20	Details	Beaver, Mercer	none
Lebanon	1813 Feb 16	Details	Dauphin, Lancaster	none
Lehigh	1812 Mar 06	Details	Northampton	none
Luzerne	1786 Sep 25	Details	Northumberland	Ontario, Susquehanna 1810, Schuylkill 1818, Wyoming 1842, Lackawanna 1878
Lycoming	1795 Apr 13	Details	Northumberland	Armstrong, Venango, Warren 1800, Indiana 1803, Clearfield, Jefferson, McKean, Potter, Tioga 1804, Ontario 1810, Clinton 1839, Sullivan 1847
McKean	1804 Mar 26	Details	Lycoming (Attached to Centre Co. 1804-1814, then to Lycoming Co. and Potter Co. until organized in 1826.)	Elk 1843, Cameron 1860
Mercer	1800 Mar 12	Details	Allegheny (Attached to Crawford Co.; organized in 1804.)	Lawrence 1849
Mifflin	1789 Sep 19	Details	Cumberland, Northumberland	Centre 1800, Juniata 1831
Monroe	1836 Apr 01	Details	Northampton, Pike	Carbon 1843.
Montgomery	1784 Sep 10	Details	Philadelphia	none
Montour	1850 May 03	Details	Columbia	none
Northampton	1752 Oct 14	Details	Bucks	Northumberland 1772, Wayne 1798, Schuylkill 1811, Lehigh 1812, Monroe 1836, Carbon 1843
Northumberland	1772 Mar 21	Details	Lancaster, Cumberland, Berks, Bedford, Northampton	Luzerne 1786, Mifflin 1789, Lycoming 1795, Centre 1800, Columbia and Union 1813.
Ontario	1810 Feb 21	Details	Luzerne	renamed Bradford 1812
Perry	1820 Mar 22	Details	Cumberland	none
Philadelphia	1682 Mar 10	Details	Colonial Lands	Berks 1751, Montgomery 1784. In 1854 all townships, districts and other parts of county consolidated into the City of Philadelphia.
Pike	1814 Mar 26	Details	Wayne	Monroe 1836
Potter	1804 Mar 26	Details	Lycoming (Attached to Lycoming Co. but elected joint commissioners with McKean Co. in 1814; became separate county in 1824; organized in 1835.)	Cameron 1860
Schuylkill	1811 Mar 01	Details	Berks, Northampton. In March 1818 additional land was received from parts of Columbia and Luzerne counties.	none
Snyder	1855 Mar 02	Details	Union	none
Somerset	1795 Apr 17	Details	Bedford	Cambria 1804
Sullivan	1847 Mar 15	Details	Lycoming	none
Susquehanna	1812 Feb 21	Details	Luzerne	none
Tioga	1804 Mar 26	Details	Lycoming (Tioga Co. was part of land claimed by Connecticut.	none
Union	1813 Mar 22	Details	Northumberland	Snyder 1855
Venango	1800 Mar 12	Details	Allegheny, Lycoming (Attached to Crawford Co.; organized in 1805.)	Clarion 1839, Forest 1866
Warren	1800 Mar 12	Details	Allegheny, Lycoming (organized in 1819. Attached to Crawford Co. until 1805, the to Venango Co. until 1819.)	none
Washington	1781 Mar 28	Details	Westmoreland	Parts to Allegheny 1788, Greene 1796, Beaver 1800
Wayne	1798 Mar 21	Details	Northampton	Pike 1814
Westmoreland	1773 Feb 26	Details	Bedford	Washington 1781, Fayette 1783, Allegheny 1788, Armstrong 1800, Indiana 1803
Wyoming	1842 Apr 04	Details	Luzerne	none
York	1749 Oct 14	Details	Lancaster	Adams 1800

Extinct or Renamed Counties: | Ontario