| County | FIPS Code | County Seat | Established in | Population | Area (sq mi) | Area (km2) |
|---|---|---|---|---|---|---|
| Adams County | 1 | Quincy | 1825 | 65,691 | 855 | 2,214 |
| Alexander County | 3 | Cairo | 1819 | 6,060 | 235 | 609 |
| Bond County | 5 | Greenville | 1817 | 16,630 | 380 | 984 |
| Boone County | 7 | Belvidere | 1837 | 53,577 | 280 | 725 |
| Brown County | 9 | Mount Sterling | 1839 | 6,556 | 305 | 790 |
| Bureau County | 11 | Princeton | 1837 | 32,993 | 869 | 2,251 |
| Calhoun County | 13 | Hardin | 1825 | 4,802 | 253 | 655 |
| Carroll County | 15 | Mount Carroll | 1839 | 14,312 | 445 | 1,153 |
| Cass County | 17 | Virginia | 1837 | 12,260 | 375 | 971 |
| Champaign County | 19 | Urbana | 1833 | 209,983 | 996 | 2,580 |
| Christian County | 21 | Taylorville | 1839 | 32,661 | 709 | 1,836 |
| Clark County | 23 | Marshall | 1819 | 15,596 | 501 | 1,298 |
| Clay County | 25 | Louisville | 1824 | 13,253 | 468 | 1,212 |
| Clinton County | 27 | Carlyle | 1824 | 37,639 | 474 | 1,228 |
| Coles County | 29 | Charleston | 1830 | 50,885 | 508 | 1,316 |
| Cook County | 31 | Chicago | 1831 | 5,180,493 | 944 | 2,445 |
| Crawford County | 33 | Robinson | 1816 | 18,807 | 443 | 1,147 |
| Cumberland County | 35 | Toledo | 1843 | 10,808 | 345 | 894 |
| DeKalb County | 37 | Sycamore | 1837 | 104,143 | 631 | 1,634 |
| DeWitt County | 39 | Clinton | 1839 | 15,769 | 397 | 1,028 |
| Douglas County | 41 | Tuscola | 1859 | 19,479 | 416 | 1,077 |
| DuPage County | 43 | Wheaton | 1839 | 928,589 | 327 | 847 |
| Edgar County | 45 | Paris | 1823 | 17,360 | 623 | 1,614 |
| Edwards County | 47 | Albion | 1814 | 6,392 | 222 | 575 |
| Effingham County | 49 | Effingham | 1831 | 34,208 | 478 | 1,238 |
| Fayette County | 51 | Vandalia | 1821 | 21,416 | 716 | 1,854 |
| Ford County | 53 | Paxton | 1859 | 13,264 | 485 | 1,256 |
| Franklin County | 55 | Benton | 1818 | 38,701 | 408 | 1,057 |
| Fulton County | 57 | Lewistown | 1823 | 34,844 | 865 | 2,240 |
| Gallatin County | 59 | Shawneetown | 1812 | 5,058 | 322 | 834 |
| Greene County | 61 | Carrollton | 1821 | 13,044 | 543 | 1,406 |
| Grundy County | 63 | Morris | 1841 | 50,972 | 418 | 1,083 |
| Hamilton County | 65 | McLeansboro | 1821 | 8,163 | 434 | 1,124 |
| Hancock County | 67 | Carthage | 1825 | 17,844 | 793 | 2,054 |
| Hardin County | 69 | Elizabethtown | 1839 | 3,910 | 177 | 458 |
| Henderson County | 71 | Oquawka | 1841 | 6,709 | 378 | 979 |
| Henry County | 73 | Cambridge | 1825 | 49,090 | 822 | 2,129 |
| Iroquois County | 75 | Watseka | 1833 | 27,604 | 1,117 | 2,893 |
| Jackson County | 77 | Murphysboro | 1816 | 57,419 | 584 | 1,513 |
| Jasper County | 79 | Newton | 1831 | 9,611 | 494 | 1,279 |
| Jefferson County | 81 | Mount Vernon | 1819 | 37,820 | 571 | 1,479 |
| Jersey County | 83 | Jerseyville | 1839 | 21,847 | 369 | 956 |
| Jo Daviess County | 85 | Galena | 1827 | 21,366 | 600 | 1,554 |
| Johnson County | 87 | Vienna | 1812 | 12,456 | 343 | 888 |
| Kane County | 89 | Geneva | 1836 | 534,216 | 519 | 1,344 |
| Kankakee County | 91 | Kankakee | 1853 | 110,024 | 676 | 1,751 |
| Kendall County | 93 | Yorkville | 1841 | 127,915 | 320 | 829 |
| Knox County | 95 | Galesburg | 1825 | 50,112 | 716 | 1,854 |
| Lake County | 97 | Waukegan | 1839 | 700,832 | 443 | 1,147 |
| LaSalle County | 99 | Ottawa | 1831 | 109,430 | 1,135 | 2,940 |
| Lawrence County | 101 | Lawrenceville | 1821 | 15,765 | 372 | 963 |
| Lee County | 103 | Dixon | 1839 | 34,223 | 724 | 1,875 |
| Livingston County | 105 | Pontiac | 1837 | 35,761 | 1,043 | 2,701 |
| Logan County | 107 | Lincoln | 1839 | 28,925 | 618 | 1,601 |
| Macon County | 115 | Decatur | 1829 | 104,712 | 580 | 1,502 |
| Macoupin County | 117 | Carlinville | 1829 | 45,313 | 862 | 2,233 |
| Madison County | 119 | Edwardsville | 1812 | 264,461 | 716 | 1,854 |
| Marion County | 121 | Salem | 1823 | 37,620 | 572 | 1,481 |
| Marshall County | 123 | Lacon | 1839 | 11,534 | 386 | 1,000 |
| Mason County | 125 | Havana | 1841 | 13,565 | 539 | 1,396 |
| Massac County | 127 | Metropolis | 1843 | 14,080 | 237 | 614 |
| McDonough County | 109 | Macomb | 1826 | 29,955 | 589 | 1,526 |
| McHenry County | 111 | Woodstock | 1836 | 308,570 | 603 | 1,562 |
| McLean County | 113 | Bloomington | 1830 | 172,828 | 1,183 | 3,064 |
| Menard County | 129 | Petersburg | 1839 | 12,288 | 314 | 813 |
| Mercer County | 131 | Aledo | 1825 | 15,601 | 561 | 1,453 |
| Monroe County | 133 | Waterloo | 1816 | 34,335 | 385 | 997 |
| Montgomery County | 135 | Hillsboro | 1821 | 28,601 | 703 | 1,821 |
| Morgan County | 137 | Jacksonville | 1823 | 33,976 | 568 | 1,471 |
| Moultrie County | 139 | Sullivan | 1843 | 14,717 | 335 | 868 |
| Ogle County | 141 | Oregon | 1836 | 50,923 | 758 | 1,963 |
| Peoria County | 143 | Peoria | 1825 | 180,621 | 618 | 1,601 |
| Perry County | 145 | Pinckneyville | 1827 | 21,174 | 441 | 1,142 |
| Piatt County | 147 | Monticello | 1841 | 16,396 | 439 | 1,137 |
| Pike County | 149 | Pittsfield | 1821 | 15,611 | 831 | 2,152 |
| Pope County | 151 | Golconda | 1816 | 4,212 | 368 | 953 |
| Pulaski County | 153 | Mound City | 1843 | 5,463 | 199 | 515 |
| Putnam County | 155 | Hennepin | 1825 | 5,740 | 160 | 414 |
| Randolph County | 157 | Chester | 1795 | 32,106 | 575 | 1,489 |
| Richland County | 159 | Olney | 1841 | 15,763 | 360 | 932 |
| Rock Island County | 161 | Rock Island | 1831 | 143,477 | 427 | 1,106 |
| Saline County | 165 | Harrisburg | 1847 | 23,906 | 379 | 982 |
| Sangamon County | 167 | Springfield | 1821 | 195,348 | 868 | 2,248 |
| Schuyler County | 169 | Rushville | 1825 | 6,907 | 437 | 1,132 |
| Scott County | 171 | Winchester | 1839 | 4,926 | 250 | 647 |
| Shelby County | 173 | Shelbyville | 1827 | 21,741 | 758 | 1,963 |
| St. Clair County | 163 | Belleville | 1790 | 261,059 | 657 | 1,702 |
| Stark County | 175 | Toulon | 1839 | 5,427 | 288 | 746 |
| Stephenson County | 177 | Freeport | 1837 | 44,753 | 564 | 1,461 |
| Tazewell County | 179 | Pekin | 1827 | 132,328 | 646 | 1,673 |
| Union County | 181 | Jonesboro | 1818 | 16,841 | 413 | 1,070 |
| Vermilion County | 183 | Danville | 1826 | 76,806 | 898 | 2,326 |
| Wabash County | 185 | Mount Carmel | 1824 | 11,549 | 223 | 578 |
| Warren County | 187 | Monmouth | 1825 | 17,032 | 542 | 1,404 |
| Washington County | 189 | Nashville | 1818 | 13,995 | 562 | 1,456 |
| Wayne County | 191 | Fairfield | 1819 | 16,332 | 713 | 1,847 |
| White County | 193 | Carmi | 1815 | 13,665 | 494 | 1,279 |
| Whiteside County | 195 | Morrison | 1836 | 55,626 | 684 | 1,772 |
| Will County | 197 | Joliet | 1836 | 692,310 | 835 | 2,163 |
| Williamson County | 199 | Marion | 1839 | 67,056 | 420 | 1,088 |
| Winnebago County | 201 | Rockford | 1836 | 284,081 | 513 | 1,329 |
| Woodford County | 203 | Eureka | 1841 | 38,463 | 527 | 1,365 |
