Admitted to the Union on July 3, 1890 as the 43rd state. Prior to statehood, the portion above 46° north was enumerated as part of Washington Territory in 1860. Idaho became a territory in 1863.
- 1870—1940 federal censuses extant for all counties except for 1890.
| Ada | Adams | Bannock | Bear Lake |
| Benewah | Bingham | Blaine | Boise |
| Bonner | Bonneville | Boundary | Butte |
| Camas | Canyon | Caribou | Cassia |
| Clark | Clearwater | Custer | Elmore |
| Franklin | Fremont | Gem | Gooding |
| Idaho | Jefferson | Jerome | Kootenai |
| Latah | Lemhi | Lewis | Lincoln |
| Madison | Minidoka | Nez Perce | Oneida |
| Owyhee | Payette | Power | Shoshone |
| Teton | Twin Falls | Valley | Washington |