An act of the Illinois General Assembly separated Putnam County from Cook County on January 15, 1831. The newly formed county was Illinois' 54th, named for Daniel Cook, one of its early and most youthful statesmen. Also, Cook was the state's first Attorney General and second U.S. Representative. Cook County was divided into DuPage County in 1839.

Since the formation of DuPage County, the shape of Cook County and its bordering counties has remained unchanged. Yet, each county's population and the proportion of farmland against residential and industrial activities has shifted considerably over the decades.

Per the U.S. Census Bureau's report, the county's population began at 10,201 in the 1840 census and increased to 5,150,233 in 2019. During the 1830s and 1840s, farmers bought up the majority of the county's accessible land and began cultivating crops and livestock. Since there were no railroads, some farmers transported their crops to Chicago, while others traveled to smaller towns closer to home.

Cook County is now home to 134 municipalities, the most well-known of which is the City of Chicago, also serving as the county seat and home to Cook County's administrative buildings.

Daniel Pope Cook, one of Illinois' first, youngest, and most talented statesmen, is commemorated in Cook County. Daniel Pope Cook was born into a poor branch of the prestigious Pope family of Kentucky and Virginia in Scott County, Kentucky.In 1815, Cook relocated to Kaskaskia, Illinois, and accepted a job as a store clerk, but he soon began studying law under his uncle, Nathaniel Pope.

He was a politician, lawyer, and newspaper proprietor from Illinois. Cook was Illinois state attorney general from 1819 to 1827 and U.S. Representative from Illinois at-large from 1819 to 1827. He died on October 16, 1827. During his political life, Cook was the state's first attorney general, then a congressman, and an anti-slavery activist.

Cook County was named after him four years after his death. In 1866, he was reinterred in Oak Ridge Cemetery in Springfield, Illinois, with the Lamb family (relatives of his son's bride).