Cook County, Illinois

"Cook County" redirects here. For other uses, see Cook County (disambiguation).

Cook County
County
Aerial view of Chicago Evanston and the lakeshore Cook County Forest Preserves Montrose Beach Frank Lloyd Wright Studio Cook County Circuit Court at Daley Plaza with Chicago Picasso
Flag Seal
Location within Illinois
Illinois's location within the United States
Coordinates: 41°48′31″N 87°53′20″W
Country	United States
State	Illinois
Region	Northern Illinois
Metro area	Chicago metropolitan area
Incorporated	January 15, 1831 (1831-01-15)
Named for	Daniel Pope Cook
County seat	Chicago
Incorporated municipalities	135 (total) 23 cities, 1 town, 111 villages (located entirely or partially within county boundaries)
Government
• Type	County commission
• Body	Board of Commissioners
• President	Toni R. Preckwinkle (D)
Area
• County	1,635 sq mi (4,230 km2)
• Land	945 sq mi (2,450 km2)
• Water	690 sq mi (1,800 km2)  42.2%
• Metro	10,874 sq mi (28,160 km2)
• Rank	6th largest county in Illinois
Highest elevation	950 ft (290 m)
Lowest elevation	580 ft (180 m)
Population  (2020)
• County	5,275,541
• Estimate  (2025)	5,194,625
• Density	5,580/sq mi (2,160/km2)
GDP
• County	$546.440 billion (2024)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (Central)
ZIP Code prefixes	600xx–608xx
Area codes	847/224, 312/872, 773/872, 708/464
FIPS code	17-031
GNIS feature ID	1784766
Congressional districts	1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th
Website	cookcountyil.gov

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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.

There are more than 800 local governmental units and nearly 130 municipalities located wholly or partially within Cook County, the largest of which is Chicago. The city is home to approximately 54 percent of the entire county's population. The part of the county outside of the Chicago and Evanston city limits is divided into 29 townships; these often divide or share governmental services with local municipalities. Townships within Chicago were abolished in 1902 but are retained for real estate assessment purposes. Evanston Township was formerly coterminous with the City of Evanston but was abolished in 2014. County government is overseen by the Cook County Board of Commissioners, with its president as chief executive, Cook County Clerk and Cook County Treasurer. Countywide state government offices include the Circuit Court of Cook County, the Cook County State's Attorney, the Cook County Sheriff, and the Cook County Assessor.

Geographically, the county is the sixth-largest in Illinois by land area and the largest by total area. It shares the state's Lake Michigan shoreline with Lake County. Including its lake area, Cook County has a total area of 1,635 square miles (4,234.6 km²), the largest county in Illinois, of which 945 square miles (2,447.5 km²) is land and 690 square miles (1,787.1 km²) (42.16%) is water. Land-use in Cook County is mostly urban and densely populated. Within Cook County, the state of Illinois took advantage of its Lake Michigan access and the Chicago Portage, beginning with the construction of the Illinois and Michigan Canal in 1848. This helped make the region a central transit hub for the nation. Chicago, with its location on the Great Lakes and via the St. Lawrence Seaway, is a global port city, giving Cook County an international shipping port.

Cook County's population is larger than that of 28 U.S. states and territories, and larger than the population of 11 of the 13 Canadian provinces and territories. The county is at the center of the Chicago metropolitan area, which has a population of approximately 10 million people.