Admitted to the Union on March 1, 1803 as the 17th state.
- No federal census taken in the Northwest Territory in 1790.
- 1800 federal census lost for all counties except Washington.
- 1810 federal census lost for all counties except Washington, and lost for Erie District, Michigan Territory.
- 1820 federal census extant for all counties except Franklin and Wood, and extant for Monroe in Michigan Territory.
- 1830 federal census extant for all counties, including Lenawee and Monroe in Michigan Territory.
- 1840 – 1940 federal censuses extant for all counties except for 1890.
| Adams | Allen | Ashland | Ashtabula |
| Athens | Auglaize | Belmont | Brown |
| Butler | Carroll | Champaign | Clark |
| Clermont | Clinton | Columbiana | Coshocton |
| Crawford | Cuyahoga | Darke | Defiance |
| Delaware | Erie | Fairfield | Fayette |
| Franklin | Fulton | Gallia | Geauga |
| Greene | Guernsey | Hamilton | Hancock |
| Hardin | Harrison | Henry | Highland |
| Hocking | Holmes | Huron | Jackson |
| Jefferson | Knox | Lake | Lawrence |
| Licking | Logan | Lorain | Lucas |
| Madison | Mahoning | Marion | Medina |
| Meigs | Mercer | Miami | Monroe |
| Montgomery | Morgan | Morrow | Muskingum |
| Noble | Ottawa | Paulding | Perry |
| Pickaway | Pike | Portage | Preble |
| Putnam | Richland | Ross | Sandusky |
| Scioto | Seneca | Shelby | Stark |
| Summit | Trumbull | Tuscarawas | Union |
| Van Wert | Vinton | Warren | Washington |
| Wayne | Williams | Wood | Wyandot |