1850 Counties of Illinois
| County | Description | Area in sq miles | Courts held at | Pop in 1850 |
| Adams | westernmost part on Mississippi river | 768 | Quincy | 26,508 |
| Alexander | southern most at junction of Mississippi & Ohio Rivers | 390 s | Unity | 2,484 |
| Boone | north boundary, drained by Kishwaukee river | 400 | Belvidere | 7,626 |
| Brown | western part, Illinois river on east & Crooked river on northeast | 300 | Mount Sterling | 7,198 |
| Bureau | northern part, northwest side of Illinois river, water by Bureau creek & Winnebago lake | 648 | Princeton | 8,841 |
| Calhoun | western boundary, between Illinois & Mississipi rivers | 240 | Gilead | 3,231 |
| Carroll | western boundary, on east side of Mississippi river | 445 | Mount Carroll | 4,586 |
| Cass | western part, on southeast side of Illinois river | 256 | Virginia | 3,253 |
| Champaign | eastern part, drained by Kaskaskia, Embarrass, Sangamon & Vermilion rivers | 792 | Urbana | 2,649 |
| Christian | central part | 350 | Edinburgh | 3,203 |
| Clark | eastern boundary, west side of Wabash river | 500 | Marshall | 9,532 |
| Clay | southeast part | 475 | Lewisville | 4,289 |
| Clinton | southern part, crossed by Kaskaskia river | 480 | Carlyle | 5,139 |
| Coles | eastern part | 1,248 | Charleston | 9,335 |
| Cook | eastern boundary, on west side of Lake Michigan & water by DesPlaines, Chicago & Du Page rivers | 864 | Chicago | 43,385 |
| Crawford | eastern boundary, Wabash river on east & Embarrass river on southwest | 426 | Palestine | 7,135 |
| Cumberland | eastern part | blank | Greenup | 3,720 |
| De Kalb | northern part | 648 | Sycamore | 7,450 |
| De Witt | central part | blank | Clinton | 5,002 |
| Du Page | northeastern part | 396 | Naperville | 9,290 |
| Edgar | eastern boundary | 630 | Paris | 10,692 |
| Edwards | southeastern part | 200 | Albion | 3,524 |
| Effingham | southeastern part | 486 | Ewington | 3,799 |
| Fayette | central part, crossed by Kaskaskia river | 648 | Vandalia | 8,075 |
| Franklin | southern part | 866 | Benton | 5,681 |
| Fulton | western part, Illinois river on southeast side | 874 | Lewiston | 22,508 |
| Gallatin | eastern boundary, Ohio & Wabash rivers on east | 760 | Equality | 5,448 |
| Greene | western part, Illinois river on west | 912 | Carrollton | 3,658 |
| Grundy | northeastern part, crossed by Des Plaines river | 324 | Morris | 3,006 |
| Hamilton | south eastern part | 432 | McLeansborough | 6,362 |
| Hancock | western boundary, Mississippi river on west | 775 | Carthage | 14,652 |
| Hardin | southeastern part, Ohio river on east & south side | 100 | Elizabethtown | 2,887 |
| Henderson | western boundary, Mississippi river on west | blank | Oquawka | 4,612 |
| Henry | north western part, Rock river on northwest | 840 | Cambridge | 3,807 |
| Highland | no description | blank | Elm Grove | blank |
| Iroquis | east boundary, crossed by Iroquis river | 1428 | Middleport | 4,149 |
| Jackson | southwest boundary on Mississippi river | 576 | Brownsville | 5,862 |
| Jasper | southeast part | 475 | Newton | 3,220 |
| Jefferson | southern part | 576 | Mount Vernon | 8,109 |
| Jersey | not listed in book as a county | (Jerseyville) | ||
| Jo-Daviess | northwest corner on Mississippi river | 724 | Galena | 18,604 |
| Johnson | southern part | 486 | Vienna | 4,113 |
| Kane | northeast part, crossed by Fox river | 1296 | Geneva | 16,7013 |
| Kendall | north eastern part, crossed by Fox river | 324 | Yorkville | 7,730 |
| Knox | west part | 792 | Knoxville | 13,279 |
| Lake | northeast corner on Lake Michigan | 425 | Waukegan | 14,226 |
| La Salle | northern part, crossed by Illinois river | 1864 | Ottawa | 17,815 |
| Lawrence | south eastern boundary, Wabash river on southeast | 560 | Lawrenceville | 6,122 |
| Lee | north part, crossed by Rock river | 720 | Dixon | 5,292 |
| Livingston | northeast part | 1026 | Pontiac | 1,552 |
| Logan | central part | 529 | Postville | 5,128 |
| Macon | central part | 1400 | Decatur | 3,988 |
| Macoupin | toward western boundary, water by Macoupin river | 800 | Carlinville | 12,355 |
| Madison | western boundary on Mississippi river | 760 | Edwardsville | 20,436 |
| Marion | toward south part | 576 | Salem | 6,720 |
| Marshall | toward north part, crossed by Illinois river | 384 | Lacon | 5,180 |
| Mason | central part, Illinois river on northwest, Sangamon river on south | blank | Bath | 5,922 |
| Massac | southern boundary, Ohio river on south | blank | Metropolis | 4,092 |
| McDonough | western part | 576 | Macomb | 7,615 |
| McHenry | northern boundary | 960 | Dorr | 14,979 |
| McLean | central part | 1296 | Bloomington | 10,163 |
| Menard | central part, crossed by Sangamon river | 260 | Petersburgh | 6,349 |
| Mercer | western boundary, Mississippi river on west | 550 | Millersburgh | 5,246 |
| Monroe | southwest boundary, Mississippi river on west & Kaskaskia river on east | 360 | Waterloo | 7,679 |
| Montgomery | towards southern part | 684 | Hillsborough | 6,276 |
| Morgan | western part, Illinois river on west | 510 | Jacksonville | 16,064 |
| Moultrie | central part, water by Kaskaskia river | not listed | not listed | 3,234 |
| Ogle | northern part, crossed by Rock river | 625 | Oregon City | 10,020 |
| Peoria | toward west part, Illinois river on southeast | 648 | Peoria | 17,547 |
| Perry | south part | 432 | Pinckeyville | 5,278 |
| Piatt | towards east part | 440 | Monticello | 1,606 |
| Pike | southwestern boundary, Mississippi river on southwest & Illinois river on east | 800 | Pittsfield | 9,095 |
| Pope | on southern boundary, Ohio river on southeast | 576 | Golconda | 3,975 |
| Putnam | north part, crossed by Illinois river | 325 | Hennepin | 3,924 |
| Randolph | southwestern boundary, Mississippi river on southwest, crossed by Kaskaskia river | 540 | Kaskaskia | 11,079 |
| Richland | south eastern part | blank | Olney | 4,012 |
| Rock Island | on northwestern boundary on Mississippi river, crossed by Rock river | blank | Rock Island | 6,937 |
| St Clair | on south western boundary, Mississippi river on west, crossed by Kaskaskia river | 648 | Bellville | 20,181 |
| Saline | southern part | blank | Raleigh | 5,558 |
| Sangnom or Sangamo | central part, crossed by Sangamon river | 900 | Springfield (Capital) | 19,228 |
| Schuyler | western part, Illinois river on southeast | 360 | Rushville | 10,573 |
| Scott | western part, Illinois river on west | 240 | Winchester | 7,914 |
| Shelby | central part, crossed by Kaskaskia river | 1080 | Shelbyville | 7,807 |
| Stark | toward northwest part, crossed by Spoon river | 288 | Toulon | 3,710 |
| Stephenson | on north boundary, crossed by Pekatonica river | 500 | Freeport | 11,666 |
| Tazewell | central part, Illinois river on northwest | 1130 | Tremont | 12,052 |
| Union | on southwestern boundary, Mississippi river on southwest | 380 | Jonesboro | 7,615 |
| Vermilion | on east boundary | 1000 | Danville | 11,492 |
| Wabash | on southeastern boundary, Wabash river on southwest | 180 | Mount Carmel | 4,690 |
| Warren | in western part | 600 | Monmouth | 8,176 |
| Washington | toward southern part, Kaskaskia river on northwest | 656 | Nashville | 6,953 |
| Wayne | in southeast part, Little Wabash on east | 720 | Fairfield | 6,828 |
| White | on southeastern boundary, Wabash river on east | 480 | Carmi | 8,925 |
| Whitesides | on northwest boundary, Mississippi river on west, crossed by Rock river | 770 | Lyndon | 5,361 |
| Will | on east boundary, crossed by Des Plaines river | 504 | Joliet | 16,703 |
| Williamson | in southern part | 432 | Marion | 7,216 |
| Winnebago | northern boundary, crossed by rock river | 504 | Rockford | 11,773 |
| Woodford | in central part, Illinois river on northwest | 470 | Woodford | 4,416 |