Admitted to the Union on September 9, 1850 as the 31st state.
- 1850 federal census extant for all counties, except Contra Costa, San Francisco, and Santa Clara.
- 1860 – 1940 federal censuses extant for all counties except for 1890.
| Alameda | Alpine | Amador | Butte |
| Calaveras | Colusa | Contra Costa | Del Norte |
| El Dorado | Fresno | Glenn | Humboldt |
| Imperial | Inyo | Kern | Kings |
| Lake | Lassen | Los Angeles | Madera |
| Marin | Mariposa | Mendocino | Merced |
| Modoc | Mono | Monterey | Napa |
| Nevada | Orange | Placer | Plumas |
| Riverside | Sacramento | San Benito | San Bernardino |
| San Diego | San Francisco | San Joaquin | San Luis Obispo |
| San Mateo | Santa Barbara | Santa Clara | Santa Cruz |
| Shasta | Sierra | Siskiyou | Solano |
| Sonoma | Stanislaus | Sutter | Tehama |
| Trinity | Tulare | Tuolumne | Ventura |
| Yolo | Yuba |