Illinois county map

How Illinois’s counties are laid out

Illinois has 102 counties in 57,914 square miles, an average of about 570 square miles each — roughly the size of a township grid of 16 by 36 miles, which is what many of the flat prairie counties actually are. The oldest counties are in the south, along the Mississippi and Ohio rivers: St. Clair and Randolph were organised in 1790 and 1795 under the Northwest Territory, before Illinois Territory existed. Settlement moved north up the rivers, and Cook County, now home to Chicago, was not created until 1831. The northern tier of counties along the Wisconsin line was the last to be organised, in 1836–1841.

Illinois has a distinctive split in local government. In 85 counties the county is subdivided into civil townships with their own elected officials; the 17 counties without townships, mainly in the south (Alexander, Calhoun, Edwards, Hardin, Johnson, Massac, Monroe, Morgan, Perry, Pope, Pulaski, Randolph, Scott, Union, Wabash, Williamson and Menard), are governed by county commissioners instead. Illinois has more units of local government than any other state.

The river boundaries matter for reading the map. The western edge of every county from Jo Daviess down to Alexander is the Mississippi River; the southern tip from Gallatin round to Alexander follows the Ohio; the Wabash forms the border with Indiana from Clark County south. The Illinois River cuts diagonally across the state from the Chicago area to Grafton, and half a dozen counties (Calhoun, Jersey, Greene, Scott, Cass, Mason) use it as a boundary. Calhoun County, a peninsula between the Illinois and the Mississippi, was reachable only by ferry until 1968.

All 102 counties of Illinois

Five county facts

• Largest by area: McLean County, seat Bloomington, about 1,180 square miles of prairie in the exact centre of the state. LaSalle County on the Illinois River is a close second.
• Smallest by area: Putnam County, seat Hennepin, about 160 square miles, tucked into a bend of the Illinois River between Bureau and LaSalle counties.
• Most populous: Cook County, seat Chicago, with about 5.28 million people in 2020 — the second most populous county in the United States after Los Angeles County, and home to about 41 per cent of Illinois. The five “collar counties” around it (DuPage, Lake, Will, Kane and McHenry) each have more than 300,000 people; DuPage alone is above 900,000.
• Least populous: Hardin County, seat Elizabethtown, on the Ohio River in the Shawnee Hills, with fewer than 4,000 people. Pope, Calhoun, Scott and Pulaski counties are also under 6,000.
• Oldest and newest: St. Clair County, organised in 1790 from the whole Illinois country, is the oldest; Randolph (1795) is second. Douglas County, created in 1859 from Coles, is the newest. Illinois has no independent cities; Chicago is a city within Cook County, though it is larger than the county government in budget and staff.

Reading the map

Press Streets and zoom in to see county lines. The most useful landmarks are the rivers: Cook and Lake counties on Lake Michigan in the north-east; the Illinois River angling south-west past Peoria and Havana; the Mississippi bulging west at Adams and Pike counties; and the wedge of Alexander County at Cairo where the Ohio meets the Mississippi — the southernmost point of the state, further south than Richmond, Virginia. Sangamon County holds the capital Springfield, Champaign County the University of Illinois, and Winnebago County the third city, Rockford. All of Illinois is on Central Time.

Related maps

• Illinois map — cities, borders and geography.
• Blank Illinois map and printable Illinois map.
• Springfield, the state capital, in Sangamon County; Chicago in Cook County.
• Neighbouring county maps: Wisconsin, Indiana, Kentucky, Missouri, Iowa.
• East North Central states · the Midwest · Great Lakes region · Central Time Zone.
• Geography: Mississippi River, Ohio River, Great Lakes.
• All US county maps.