1850 Counties of Illinois
| in area | Courts held at | Pop in 1850 | ||
| Adams Co, IL | westernmost part on Mississippi river | 768 sq miles | Quincy | 26,508 |
| Alexander Co, IL | southern most at junction of Mississippi & Ohio Rivers | 390 sq miles | Unity | 2,484 |
| Boone Co, IL | north boundary, drained by Kishwaukee river | 400 sq miles | Belvidere | 7,626 |
| Brown Co, IL | western part, Illinois river on east & Crooked river on northeast | 300 sq miles | Mount Sterling | 7,198 |
| Bureau Co, IL | northern part, northwest side of Illinois river, water by Bureau creek & Winnebago lake | 648 sq miles | Princeton | 8,841 |
| Calhoun Co, IL | western boundary, between Illinois & Mississipi rivers | 240 | Gilead | 3,231 |
| Carroll Co, IL | western boundary, on east side of Mississippi river | 445 | Mount Carroll | 4,586 |
| Cass Co, IL | western part, on southeast side of Illinois river | 256 | Virginia | 3,253 |
| Champaign Co, IL | eastern part, drained by Kaskaskia, Embarrass, Sangamon & Vermilion rivers | 792 | Urbana | 2,649 |
| Christian Co, IL | central part | 350 | Edinburgh | 3,203 |
| Clark Co, IL | eastern boundary, west side of Wabash river | 500 | Marshall | 9,532 |
| Clay Co, IL | southeast part | 475 | Lewisville | 4,289 |
| Clinton Co, IL | southern part, crossed by Kaskaskia river | 480 | Carlyle | 5,139 |
| Coles Co, IL | eastern part | 1,248 | Charleston | 9,335 |
| Cook Co, IL | eastern boundary, on west side of Lake Michigan & water by DesPlaines, Chicago & Du Page rivers | 864 | Chicago | 43,385 |
| Crawford Co, IL | eastern boundary, Wabash river on east & Embarrass river on southwest | 426 | Palestine | 7,135 |
| Cumberland Co, IL | eastern part | blank | Greenup | 3,720 |
| De Kalb Co, IL | northern part | 648 | Sycamore | 7,450 |