Admitted to the Union on December 3, 1818 as the 21st state. Prior to statehood, it was enumerated as Illinois Territory (became a territory in 1809).

• 1800 federal census lost for all counties.
• 1810 federal census extant for Randolph (southern portion of the territory) and lost for St. Clair (upper two-thirds of territory).
• 1890 federal census lost for all counties except part of McDonough (Mound Township).
• 1820—1940 federal censuses extant for all counties except for 1890 (see above).