The largest county in Illinois
There are two right answers. By land area it is McLean County; by population it is Cook County. Which one you want depends on what you meant by largest.
Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025. Retrieved 30 August 2026.
By land area
| Rank | Name | Land area | Population |
|---|---|---|---|
| 1 | McLean County | 1,183 sq mi | 171,419 |
| 2 | LaSalle County | 1,135 sq mi | 107,773 |
| 3 | Iroquois County | 1,117 sq mi | 26,150 |
| 4 | Livingston County | 1,044 sq mi | 35,057 |
| 5 | Champaign County | 996.1 sq mi | 209,972 |
| 6 | Cook County | 944.9 sq mi | 5,194,625 |
| 7 | Vermilion County | 898.3 sq mi | 71,259 |
| 8 | Bureau County | 869.1 sq mi | 32,510 |
| 9 | Sangamon County | 868.2 sq mi | 194,170 |
| 10 | Fulton County | 865.7 sq mi | 32,901 |
By population
| Rank | Name | Population | Density |
|---|---|---|---|
| 1 | Cook County | 5,194,625 | 5,497 per sq mi |
| 2 | DuPage County | 934,298 | 2,849 per sq mi |
| 3 | Lake County | 719,339 | 1,621 per sq mi |
| 4 | Will County | 712,253 | 852 per sq mi |
| 5 | Kane County | 525,757 | 1,012 per sq mi |
| 6 | McHenry County | 317,751 | 527 per sq mi |
| 7 | Winnebago County | 283,674 | 553 per sq mi |
| 8 | Madison County | 263,110 | 368 per sq mi |
| 9 | St. Clair County | 250,708 | 381 per sq mi |
| 10 | Champaign County | 209,972 | 211 per sq mi |
And the smallest
More on Illinois
Rebuilt from source data 30 August 2026. How these figures are produced.