Illinois County Map

How Many Counties Are in Illinois?

Illinois has 102 counties, which places it sixth nationally behind Texas with 254, Georgia with 159, Kentucky with 120, Missouri with 114, and Kansas with 105. St. Clair County came first in 1790 under the Northwest Territory, named for Arthur St. Clair, and fifteen counties existed by the time Illinois became a state in 1818. Ford County, created in 1859 and named for the state's eighth governor, is the newest. Douglas County was also created in 1859, so the ranking turns on the exact date within that year, and Ford is the conventional answer.

Which Illinois County Has the Largest Population?

Cook County had an estimated 5,194,625 residents as of July 1, 2025, which makes it the second most populous county in the United States behind Los Angeles County. That figure represents roughly 41 percent of the 12.7 million people in Illinois. Cook has lost about 1.5 percent of its residents since the 2020 census, when it counted 5,275,541. The contrast is what makes the Illinois map unusual, since Cook covers 945 square miles while the fifty smallest counties combined have well under a tenth of the state's residents.

Which Illinois County Has the Fewest Residents?

Hardin County has roughly 3,575 residents, the lowest in Illinois, and it lies at the southeastern end of the state on the Ohio River. Pope County follows closely at about 3,627, and the two are close enough for the ranking to flip between estimate vintages. Calhoun, Alexander, Gallatin, and Pulaski all fall between 4,200 and 4,800. Hardin is frequently confused with the smallest county by area, which is a different county entirely. The median Illinois county holds about 24,200 residents, so half the counties in the state have fewer than 25,000 people.

What Is the Largest County in Illinois by Area?

McLean County, in the center of the state, covers about 1,183 square miles of land, more than any other Illinois county, and Bloomington is its seat. Putnam County is the smallest at 160 square miles of land and 172 square miles of total area, with its seat at Hennepin. Hardin follows at 177 square miles, which is where the confusion with population rankings arises. Massac, Alexander, Wabash, and Edwards are all under 250 square miles. The median county covers roughly 530 square miles and the average about 544.

What Are the Collar Counties?

The collar counties are the five that ring Cook County and form the rest of the core Chicago metropolitan area. DuPage is the largest by population at about 936,000 across 328 square miles, followed by Lake at 720,000, Will at 716,000 across 836 square miles, Kane at 528,000, and McHenry at 320,000. Together they hold roughly 3.22 million residents. Kendall County is sometimes treated informally as a sixth collar county because it is in the metropolitan statistical area and leads the state in growth, though the classic definition covers only the five.

Which County Is Chicago, Aurora, or Springfield In?

Chicago is in Cook County, with a small portion of O'Hare International Airport extending into DuPage. Aurora spans four counties, with Kane as its original and primary county and portions in DuPage, Will, and Kendall. Naperville spans DuPage and Will. Joliet is the seat of Will County with a portion in Kendall, and Elgin is primarily in Kane with a portion in Cook. Springfield is the seat of Sangamon County, Rockford of Winnebago, and Peoria of Peoria. Champaign shares its county with Urbana, which holds the courthouse.

How Does County Government Work in Illinois?

Illinois counties use one of two structures. Eighty-five counties use the township form, with a county board of five to twenty-nine members working through standing committees while townships handle road maintenance, general assistance, and property assessment. Seventeen counties use the commission form, with a board of three or five commissioners acting as both the legislative and executive body and the county performing the functions townships would otherwise cover. Cook County is the outlier, with a seventeen-member board of commissioners plus a separately elected board president who serves as chief executive.

Why Does Illinois Have So Many Units of Local Government?

Illinois has more units of local government than any other state, though the exact figure depends on methodology. The 2022 Census of Governments counted 6,918, the Illinois Department of Revenue counts 6,042, the State Comptroller counts 8,529, and the Civic Federation's composite tally reaches 8,923. The 102 counties are a small fraction of that total. The bulk comes from roughly 1,400 townships, roughly 1,300 municipalities, school districts, and above all special districts covering fire protection, parks, libraries, drainage, sanitation, and mosquito abatement. In the seventeen commission counties there is no township layer at all.

How Did Illinois Counties Get Their Names?

Illinois named nearly all of its counties before the Civil War, so no county honors a Civil War figure. Presidents account for Adams, Jefferson, Madison, Monroe, Jackson, and Washington. Revolutionary War officers account for Putnam, Greene, Wayne, Marion, Montgomery, Warren, Mercer, Knox, Lee, and Pulaski. French colonial names survive in LaSalle, DuPage, Bureau, and Vermilion, and tribal names in Iroquois, Kankakee, Peoria, and Winnebago. Two counties honor the same man, since DeWitt County and Clinton County are both named for DeWitt Clinton of New York. Calhoun County, created in 1825, reflects how heavily early Illinois was settled from the South.

Which Illinois Counties Are Gaining and Losing Population?

Kendall County is the only Illinois county in double-digit growth, up 11.67 percent since the 2020 census to roughly 147,800 residents. Grundy, McHenry, Bond, Will, DeKalb, Kane, and Champaign follow at between 2 and 3.5 percent. Decline is far more common, and Alexander County at the southern tip around Cairo has lost about 12.43 percent since 2020, with Lawrence down 9.58 percent, Pulaski down 8.10 percent, and Gallatin down 7.78 percent. The large majority of the state's 102 counties held fewer residents in the latest estimates than they did in 2020.