Admitted to the Union on August 10, 1821 as the 24th state. As part of the Louisiana Purchase, Louisiana Territory was created in 1805 and Missouri Territory was created in 1812.
- 1810 – 1820 federal censuses are lost.
- 1830 – 1940 federal censuses are extant for all counties except for 1890.
| Adair | Andrew | Atchison | Audrain |
| Barry | Barton | Bates | Benton |
| Bollinger | Boone | Buchanan | Butler |
| Caldwell | Callaway | Camden | Cape Girardeau |
| Carroll | Carter | Cass | Cedar |
| Chariton | Christian | Clark | Clay |
| Clinton | Cole | Cooper | Crawford |
| Dade | Dallas | Daviess | De Kalb |
| Dent | Douglas | Dunklin | Franklin |
| Gasconade | Gentry | Greene | Grundy |
| Harrison | Henry | Hickory | Holt |
| Howard | Howell | Iron | Jackson |
| Jasper | Jefferson | Johnson | Knox |
| Laclede | Lafayette | Lawrence | Lewis |
| Lincoln | Linn | Livingston | Macon |
| Madison | Maries | Marion | McDonald |
| Mercer | Miller | Mississippi | Moniteau |
| Monroe | Montgomery | Morgan | New Madrid |
| Newton | Nodaway | Oregon | Osage |
| Ozark | Pemiscot | Perry | Pettis |
| Phelps | Pike | Platte | Polk |
| Pulaski | Putnam | Ralls | Randolph |
| Ray | Reynolds | Ripley | St. Charles |
| St. Clair | Ste. Genevieve | St. Francois | St. Louis |
| St. Louis City | Saline | Schuyler | Scotland |
| Scott | Shannon | Shelby | Stoddard |
| Stone | Sullivan | Taney | Texas |
| Vernon | Warren | Washington | Wayne |
| Webster | Worth | Wright |