| County | Year Created | Parent County/Counties |
|---|---|---|
| Adams | 1797 | Hamilton |
| Allen | 1820 | Shelby |
| Ashland | 1846 | Wayne, Richland, Huron, Lorain |
| Ashtabula | 1808 | Trumbull, Geauga |
| Athens | 1805 | Washington |
| Auglaize | 1848 | Allen, Mercer, Darke, Hardin, Logan, Shelby, Van Wert |
| Belmont | 1801 | Jefferson, Washington |
| Brown | 1818 | Adams, Clermont |
| Butler | 1803 | Hamilton |
| Carroll | 1833 | Columbiana, Stark, Harrison, Jefferson, Tuscarawas |
| Champaign | 1805 | Greene, Franklin |
| Clark | 1818 | Champaign, Madison, Greene |
| Clermont | 1800 | Hamilton |
| Clinton | 1810 | Highland, Warren |
| Columbiana | 1803 | Jefferson, Washington |
| Coshocton | 1810 | Muskingum, Tuscarawas |
| Crawford | 1820 | Delaware |
| Cuyahoga | 1808 | Geauga |
| Darke1 Darke2 | 1809 | Miami |
| Defiance | 1845 | Williams, Henry, Paulding |
| Delaware | 1808 | Franklin |
| Erie | 1838 | Huron, Sandusky |
| Fairfield | 1800 | Ross, Washington |
| Fayette | 1810 | Ross, Highland |
| Franklin | 1803 | Ross |
| Fulton | 1850 | Lucas, Henry, Williams |
| Gallia | 1803 | Washington, Adams |
| Geauga | 1806 | Trumbull |
| Greene | 1803 | Hamilton, Ross |
| Guernsey | 1810 | Belmont, Muskingum |
| Hamilton | 1790 | Original County |
| Hancock | 1820 | Logan |
| Hardin | 1820 | Logan |
| Harrison | 1813 | Jefferson, Tuscarawas |
| Henry | 1820 | Shelby |
| Highland | 1805 | Ross, Adams, Clermont |
| Hocking | 1818 | Athens, Ross, Fairfield |
| Holmes | 1824 | Coshocton, Wayne, Tuscarawas |
| Huron | 1809 | Portage, Cuyahoga |
| Jackson | 1816 | Scioto, Gallia, Athens, Ross |
| Jefferson | 1797 | Washington |
| Knox | 1808 | Fairfield |
| Lake | 1840 | Geauga, Cuyahoga |
| Lawrence | 1815 | Gallia, Scioto |
| Licking | 1808 | Fairfield |
| Logan | 1818 | Champaign |
| Lorain | 1822 | Huron, Cuyahoga, Medina |
| Lucas | 1835 | Wood, Sandusky, Henry |
| Madison | 1810 | Franklin |
| Mahoning | 1846 | Columbiana, Trumbull |
| Marion | 1820 | Delaware |
| Medina | 1812 | Portage |
| Meigs | 1819 | Gallia, Athens |
| Mercer | 1820 | Darke |
| Miami | 1807 | Montgomery |
| Monroe | 1813 | Belmont, Washington, Guernsey |
| Montgomery | 1803 | Hamilton |
| Morgan | 1817 | Washington, Guernsey, Muskingum |
| Morrow | 1848 | Knox, Marion, Delaware, Richland |
| Muskingum | 1804 | Washington, Fairfield |
| Noble | 1851 | Monroe, Washington, Morgan, Guernsey |
| Ottawa | 1840 | Erie, Sandusky, Lucas |
| Paulding | 1820 | Darke |
| Perry | 1818 | Washington, Fairfield, Muskingum |
| Pickaway | 1810 | Ross, Fairfield, Franklin |
| Pike | 1815 | Ross, Scioto, Adams |
| Portage | 1808 | Trumbull |
| Preble | 1808 | Montgomery, Butler |
| Putnam | 1820 | Shelby |
| Richland | 1808 | Fairfield |
| Ross | 1798 | Adams, Washington |
| Sandusky | 1820 | Huron |
| Scioto | 1803 | Adams, Washington |
| Seneca | 1820 | Huron |
| Shelby | 1819 | Miami |
| Stark | 1808 | Columbiana |
| Summit | 1840 | Portage, Medina, Stark |
| Trumbull | 1800 | Jefferson |
| Tuscarawas | 1808 | Muskingum |
| Union | 1820 | Franklin, Madison, Logan, Delaware |
| Van Wert | 1820 | Darke |
| Vinton | 1850 | Gallia, Athens, Ross, Jackson, Hocking |
| Warren | 1803 | Hamilton |
| Washington | 1788 | Original County |
| Wayne | 1808 | Columbiana |
| Williams | 1820 | Darke |
| Wood | 1820 | Logan |
| Wyandot | 1845 | Marion, Crawford, Hardin, Hancock |
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