Admitted to the Union on December 28, 1846 as the 28th state. Iowa became a territory in 1838.
- 1840—1940 federal censuses extant for all counties except for 1890.
| Adair | Adams | Allamakee | Appanoose |
| Audubon | Benton | Black Hawk | Boone |
| Bremer | Buchanan | Buena Vista | Butler |
| Calhoun | Carroll | Cass | Cedar |
| Cerro Gordo | Cherokee | Chickasaw | Clarke |
| Clay | Clayton | Clinton | Crawford |
| Dallas | Davis | Decatur | Delaware |
| Des Moines | Dickinson | Dubuque | Emmet |
| Fayette | Floyd | Franklin | Fremont |
| Greene | Grundy | Guthrie | Hamilton |
| Hancock | Hardin | Harrison | Henry |
| Howard | Humboldt | Ida | Iowa |
| Jackson | Jasper | Jefferson | Johnson |
| Jones | Keokuk | Kossuth | Lee |
| Linn | Louisa | Lucas | Lyon |
| Madison | Mahaska | Marion | Marshall |
| Mills | Mitchell | Monona | Monroe |
| Montgomery | Muscatine | O’Brien | Osceola |
| Page | Palo Alto | Plymouth | Pocahontas |
| Polk | Pottawattamie | Poweshiek | Ringgold |
| Sac | Scott | Shelby | Sioux |
| Story | Tama | Taylor | Union |
| Van Buren | Wapello | Warren | Washington |
| Wayne | Webster | Winnebago | Winneshiek |
| Woodbury | Worth | Wright |