Cook County was created on January 15, 1831 by an act of
the Illinois State Legislature as the 54th county established in Illinois. The
unincorporated Fort Dearborn settlement at the mouth of the Chicago River
became the new county's seat. On May 7, 1831, Cook County elected its first
Cook County is an urban county in the upper northeastern section of the State of Illinois that contains more than 800 local governmental units within its boundaries. With a population of approximately 5.2 million people, it is the second most populous county in the nation and the 19th largest government in the United States (2015 census statistics).