List of counties in Illinois in the context of Putnam County, Illinois

Cook County is the most populous county in the U.S. state of Illinois and the second-most-populous county in the United States, after Los Angeles County, California. More than 40 percent of all residents of Illinois live within Cook County. As of 2020, the population was 5,275,541. The county seat is Chicago, the most populous city in Illinois and the third most populous city in the United States. The county is at the center of the Chicago metropolitan area.

Cook County was incorporated in 1831 and named for Daniel Pope Cook, an early Illinois statesman. It achieved its present boundaries in 1839. Within a century, the county recorded explosive population growth, going from a trading post village with a little over six hundred residents to four million, rivaling Paris by the Great Depression. During the first half of the 20th century, it had the absolute majority of Illinois's population.

St. Clair County is the ninth most populous county in Illinois. Located directly east of St. Louis, the county is part of the Metro East region of the Greater St. Louis metropolitan area in southern Illinois. As of the 2020 United States census, St. Clair County had a population of 257,400, making it the second most populous county in Illinois outside the Northern Third. Belleville is the county seat and largest city.

Along the Mississippi River, Cahokia Village was founded in 1697 by French settlers and served as a Jesuit mission to convert tribes of the Illinois Confederation to Christianity. The area became the center of the French Illinois Country. Prior to the establishment of Illinois as a state, the government of the Northwest Territory created St. Clair County in 1790. In 1809, the county became the administrative center of the Illinois Territory and one of the two original counties of Illinois, alongside Randolph County. In 1970, the United States Census Bureau placed the mean center of U.S. population, which generally has moved west every decennial census, in St. Clair County.