| County | Date
Formed |
Parent County | County Seat |
|---|---|---|---|
| Adams | 1797 | Hamilton | West Union |
| Allen | 1820 | Shelby | Lima |
| Ashland | 1846 | Wayne, Richland, Huron, Lorain | Ashland |
| Ashtabula | 1808 | Trumbull, Geauga | Jefferson |
| Athens | 1805 | Washington | Athens |
| Auglaize | 1848 | Allen, Mercer, Darke, Hardin, Logan, Shelby, and Van Wert | Wapakoneta |
| Belmont | 1801 | Jefferson, Washington | Saint Clairsville |
| Brown | 1818 | Adams, Cleremont | Georgetown |
| Butler | 1803 | Hamilton | Hamilton |
| Carroll | 1833 | Columbiana, Stark, Harrison, Jefferson, Tuscarawas | Carrollton |
| Champaign | 1805 | Green, Franklin | Urbana |
| Clark | 1818 | Champaign, Madison, Greene | Springfield |
| Clermont | 1800 | Hamilton | Batavia |
| Clinton | 1810 | Highland, Warren | Wilmington |
| Columbiana | 1803 | Jefferson, Washington | Lisbon |
| Coshocton | 1811 | Muskingum, Tuscarawas | Coshocton |
| Crawford | 1820 | Delaware | Bucyrus |
| Cuyahoga | 1808 | Geauga | Cleveland |
| Darke | 1809 | Miami | Greenville |
| Defiance | 1845 | Williams, Henry, Paulding | Defiance |
| Delaware | 1808 | Franklin | Delaware |
| Erie | 1838 | Huron, Sandusky | Sandusky |
| Fairfield | 1800 | Ross, Washington | Lancaster |
| Fayette | 1810 | Ross, Highland | Washington Court House |
| Franklin | 1803 | Ross, Wayne Co., MI | Columbus |
| Fulton | 1850 | Lucas, Henry, Williams | Wauseon |
| Gallia | 1803 | Washington, Adams | Gallipolis |
| Geauga | 1806 | Trumbull | Chardon |
| Greene | 1803 | Hamilton, Ross | Xenia |
| Guernsey | 1810 | Belmont, Muskingum | Cambridge |
| Hamilton | 1790 | Original County | Cincinnati |
| Hancock | 1820 | Logan | Findlay |
| Hardin | 1820 | Logan | Kenton |
| Harrison | 1813 | Jefferson, Tuscarawas | Cadiz |
| Henry | 1820 | Shelby | Napoleon |
| Highland | 1805 | Ross, Adams, Clermont | Hillsboro |
| Hocking | 1818 | Athens, Ross, Fairfield | Logan |
| Holmes | 1824 | Coshocton, Wayne, Tuscarawas | Millersburg |
| Huron | 1815 | Portage, Cuyahoga | Norwalk |
| Jackson | 1816 | Scioto, Gallia, Athens, Ross | Jackson |
| Jefferson | 1797 | Washington | Steubenville |
| Knox | 1808 | Fairfield | Mount Vernon |
| Lake | 1840 | Geauga, Cuyahoga | Painesville |
| Lawrence | 1815 | Gallia, Scioto | Ironton |
| Licking | 1808 | Fairfield | Newark |
| Logan | 1818 | Champaign | Bellefontaine |
| Lorain | 1822 | Huron, Cuyahoga, Medina | Elyria |
| Lucas | 1835 | Wood, Sandusky, Huron | Toledo |
| Madison | 1810 | Franklin | London |
| Mahoning | 1846 | Columbiana, Trumbull | Youngstown |
| Marion | 1820 | Delaware | Marion |
| Medina | 1812 | Portage | Medina |
| Meigs | 1819 | Gallia, Athens | Pomeroy |
| Mercer | 1820 | Darke | Celina |
| Miami | 1807 | Montgomery | Troy |
| Monroe | 1813 | Belmont, Washington, Guernsey | Woodsfield |
| Montgomery | 1803 | Hamilton, Wayne Co., MI | Dayton |
| Morgan | 1817 | Washington, Guernsey, Muskingum | McConnellsville |
| Morrow | 1848 | Knox, Marion, Delaware, Richland | Mount Gilead |
| Muskingum | 1804 | Washington, Fairfield | Zanesville |
| Noble | 1851 | Monroe, Washington, Morgan, Guernsey | Caldwell |
| Ottawa | 1840 | Erie, Sandusky, Lucas | Port Clinton |
| Paulding | 1820 | Darke | Paulding |
| Perry | 1818 | Washington, Fairfield, Muskingum | New Lexington |
| Pickaway | 1810 | Ross, Fairfield, Franklin | Circleville |
| Pike | 1815 | Ross, Scioto, Adams | Waverly |
| Portage | 1808 | Trumbull | Ravenna |
| Preble | 1808 | Montgomery, Butler | Eaton |
| Putnam | 1820 | Shelby | Ottawa |
| Richland | 1808 | Fairfield | Mansfield |
| Ross | 1798 | Adams, Washington | Chillicothe |
| Sandusky | 1820 | Huron | Fremont |
| Scioto | 1803 | Adams | Portsmouth |
| Seneca | 1820 | Huron | Tiffin |
| Shelby | 1819 | Miami | Sidney |
| Stark | 1808 | Columbiana | Canton |
| Summit | 1840 | Akron | |
| Trumbull | 1800 | Warren | |
| Tuscarawas | 1808 | Muskingum | New Philadelphia |
| Union | 1820 | Franklin, Madison, Logan, Delaware | Marysville |
| Van Wert | 1820 | Darke | Van Wert |
| Vinton | 1850 | Gallia, Athens, Ross, Jackson, Hocking | Mc Arthur |
| Warren | 1803 | Hamilton | Lebanon |
| Washington | 1788 | Original County | Marietta |
| Wayne | 1808 | Columbiana | Wooster |
| Williams | 1820 | Darke | Bryan |
| Wood | 1820 | Indian Lands | Bowling Green |
| Wyandot | 1845 | Marion, Crawford, Hardin | Upper Sandusky |