Illinois actually became the 21st state on December 3, 1818..... but.....
It was on April 18, 1818, that President James Monroe signed the statehood enabling act adopted by Congress which provided for a state Constitution and established our disputed northern boundary line, which made Winnebago County a part of Illinois rather than a part of Wisconsin.
Illinois' first governor, Shadrach Bond, was inaugurated on October 6, 1818, and Illinois' first constitution was adopted on the date that appears on the state seal, Aug. 26, 1818.
Credit for being the prime mover toward Illinois statehood goes to a young man by the name of Daniel Pope Cook, who at the age of 20 became the owner and editor of the first newspaper in Illinois Territory. He renamed it the Western Intelligencer.
After serving as a special messenger to European capitals for President John Quincy Adams, Cook returned to Kaskasia on Nov. 18, 1817, and began the intensive campaign which brought statehood to Illinois a year later.
There was a federal requirement that a territory had to have a population of 60,000 before it could be granted statehood. Actual population of Illinois at that time was about 40,000, but a continuing census was taken until the 60,000 figure was reached.
Cook didn't waste any time after his return from Washington. On Nov. 17, 1817, the Intelligencer carried a bombshell of an article supporting statehood. Three days later the territorial legislature met in Kaskasia and, on Dec. 6, adopted a memorial to congress requesting action at once to give Illinois statehood. The memorial was based on the Intelligencer article.
Nathaniel Pope, the territorial representative in Congress took over as soon as the memorial reached Congress on Jan. 16, 1818. It was reported out of committee Jan. 23.
On the floor of the House on April 4, Pope amended it to move the northern boundary of the proposed new state 41 miles further to the north. Originally the boundary was only 10 miles north of the southern tip of Lake Michigan.
It was this amendment which made Winnebago County and most of 14 other counties in the Northern Illinois tier, as well as the city of Chicago part of Illinois.
Pope argued this was necessary in order to bind the proposed state together with the eastern states. His amendment was approved without objection.
With some minor amendments the act was adopted by the Senate April 14. President Monroe signed it April 18, 1818, making Illinois the 21st state in the union.