| County | Rank | Change (2020) ↓ | 2026 Pop. | 2020 Pop. |
|---|---|---|---|---|
| Kendall County | 1 | 12.62% | 149,055 | 132,351 |
| Champaign County | 2 | 5.45% | 217,496 | 206,252 |
| McHenry County | 3 | 3.56% | 321,007 | 309,973 |
| Grundy County | 4 | 3.42% | 54,331 | 52,535 |
| Will County | 5 | 3.17% | 718,983 | 696,909 |
| Kane County | 6 | 2.34% | 528,509 | 516,403 |
| DuPage County | 7 | 2.31% | 953,066 | 931,508 |
| DeKalb County | 8 | 2.31% | 102,683 | 100,368 |
| Lake County | 9 | 1.96% | 727,398 | 713,426 |
| Putnam County | 10 | 1.63% | 5,723 | 5,631 |
| Clinton County | 11 | 1.56% | 37,489 | 36,913 |
| Ford County | 12 | 1.55% | 13,721 | 13,511 |
| Brown County | 13 | 1.31% | 6,319 | 6,237 |
| McLean County | 14 | 1.16% | 173,221 | 171,236 |
| Douglas County | 15 | 1.14% | 20,006 | 19,780 |
| Jackson County | 16 | 1.12% | 53,863 | 53,264 |
| Williamson County | 17 | 0.89% | 67,718 | 67,121 |
| Winnebago County | 18 | 0.69% | 286,942 | 284,963 |
| Effingham County | 19 | 0.56% | 34,900 | 34,707 |
| Boone County | 20 | 0.23% | 53,461 | 53,339 |
| Rock Island County | 21 | -0.03% | 144,315 | 144,360 |
| Peoria County | 22 | -0.05% | 181,420 | 181,512 |
| Cook County | 23 | -0.08% | 5,262,807 | 5,267,072 |
| Moultrie County | 24 | -0.35% | 14,441 | 14,492 |
| Johnson County | 25 | -0.54% | 13,250 | 13,322 |
| Cumberland County | 26 | -0.63% | 10,371 | 10,437 |
| Wayne County | 27 | -0.79% | 16,034 | 16,161 |
| Woodford County | 28 | -0.81% | 38,096 | 38,407 |
| Sangamon County | 29 | -0.83% | 194,603 | 196,223 |
| Crawford County | 30 | -0.84% | 18,480 | 18,637 |
| Kankakee County | 31 | -0.88% | 106,458 | 107,403 |
| Livingston County | 32 | -0.93% | 35,483 | 35,816 |
| LaSalle County | 33 | -0.94% | 108,376 | 109,409 |
| Monroe County | 34 | -0.96% | 34,619 | 34,953 |
| Ogle County | 35 | -0.98% | 51,253 | 51,762 |
| Schuyler County | 36 | -1.02% | 6,800 | 6,870 |
| De Witt County | 37 | -1.09% | 15,295 | 15,464 |
| Tazewell County | 38 | -1.14% | 129,793 | 131,296 |
| Randolph County | 39 | -1.2% | 29,723 | 30,084 |
| Logan County | 40 | -1.32% | 27,538 | 27,905 |
| Madison County | 41 | -1.35% | 262,167 | 265,749 |
| Whiteside County | 42 | -1.38% | 54,807 | 55,574 |
| Piatt County | 43 | -1.43% | 16,444 | 16,683 |
| Jersey County | 44 | -1.67% | 21,134 | 21,492 |
| Coles County | 45 | -1.87% | 46,205 | 47,084 |
| Lee County | 46 | -1.89% | 33,420 | 34,063 |
| Washington County | 47 | -1.94% | 13,465 | 13,731 |
| Stark County | 48 | -1.97% | 5,282 | 5,388 |
| Knox County | 49 | -1.98% | 48,822 | 49,806 |
| Fayette County | 50 | -2.01% | 21,053 | 21,485 |
| Pope County | 51 | -2.1% | 3,679 | 3,758 |
| Carroll County | 52 | -2.1% | 15,348 | 15,678 |
| Union County | 53 | -2.15% | 16,926 | 17,298 |
| Shelby County | 54 | -2.23% | 20,440 | 20,907 |
| Bond County | 55 | -2.38% | 16,454 | 16,856 |
| Franklin County | 56 | -2.47% | 36,787 | 37,717 |
| Christian County | 57 | -2.59% | 33,103 | 33,982 |
| Scott County | 58 | -2.64% | 4,940 | 5,074 |
| Henry County | 59 | -2.79% | 47,807 | 49,178 |
| Macon County | 60 | -2.95% | 100,703 | 103,764 |
| Edgar County | 61 | -2.96% | 16,310 | 16,808 |
| Marshall County | 62 | -3.02% | 11,356 | 11,710 |
| St. Clair County | 63 | -3.04% | 249,259 | 257,070 |
| Cass County | 64 | -3.04% | 12,581 | 12,976 |
| Menard County | 65 | -3.06% | 11,897 | 12,272 |
| Iroquois County | 66 | -3.08% | 26,135 | 26,966 |
| Jasper County | 67 | -3.14% | 8,964 | 9,255 |
| Montgomery County | 68 | -3.22% | 27,333 | 28,242 |
| Adams County | 69 | -3.3% | 63,457 | 65,622 |
| Macoupin County | 70 | -3.32% | 43,431 | 44,924 |
| Morgan County | 71 | -3.54% | 32,308 | 33,494 |
| Jefferson County | 72 | -3.62% | 35,659 | 37,000 |
| Pike County | 73 | -3.7% | 14,096 | 14,637 |
| Warren County | 74 | -3.74% | 16,203 | 16,832 |
| McDonough County | 75 | -3.81% | 26,082 | 27,116 |
| Bureau County | 76 | -3.86% | 31,942 | 33,225 |
| Hamilton County | 77 | -3.89% | 7,682 | 7,993 |
| Wabash County | 78 | -3.91% | 10,873 | 11,315 |
| Fulton County | 79 | -4.03% | 32,202 | 33,554 |
| Clark County | 80 | -4.25% | 14,792 | 15,448 |
| Lawrence County | 81 | -4.27% | 14,631 | 15,283 |
| Mercer County | 82 | -4.27% | 14,964 | 15,632 |
| White County | 83 | -4.29% | 13,221 | 13,814 |
| Marion County | 84 | -4.32% | 36,031 | 37,657 |
| Stephenson County | 85 | -4.49% | 42,570 | 44,573 |
| Richland County | 86 | -4.57% | 15,082 | 15,805 |
| Jo Daviess County | 87 | -4.58% | 21,086 | 22,098 |
| Mason County | 88 | -4.6% | 12,453 | 13,053 |
| Edwards County | 89 | -4.82% | 5,910 | 6,209 |
| Vermilion County | 90 | -5.13% | 70,216 | 74,010 |
| Perry County | 91 | -5.14% | 19,818 | 20,892 |
| Saline County | 92 | -5.22% | 22,502 | 23,741 |
| Hancock County | 93 | -5.26% | 16,634 | 17,558 |
| Clay County | 94 | -5.27% | 12,573 | 13,273 |
| Hardin County | 95 | -5.32% | 3,452 | 3,646 |
| Massac County | 96 | -5.36% | 13,379 | 14,137 |
| Gallatin County | 97 | -5.88% | 4,654 | 4,945 |
| Henderson County | 98 | -6.33% | 5,934 | 6,335 |
| Greene County | 99 | -6.94% | 11,107 | 11,935 |
| Calhoun County | 100 | -7.2% | 4,086 | 4,403 |
| Pulaski County | 101 | -8.8% | 4,723 | 5,179 |
| Alexander County | 102 | -16.16% | 4,336 | 5,172 |
Illinois's counties show varied growth patterns from 2020 to 2026. Kendall County leads with 2% annualized growth, followed by Champaign County (0.89%) and McHenry County (0.58%). The largest counties are Cook County (5,262,807), DuPage County (953,066), and Lake County (727,398). Of the 102 counties in Illinois, 20 are growing while 82 have experienced population decline. Overall, counties averaged -0.35% annualized growth, with total population growing by 0.06% annually.
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