| County | Rank | Change (2020) ↓ | 2025 Pop. | 2020 Pop. |
|---|---|---|---|---|
| Kendall County | 1 | 9.51% | 144,940 | 132,355 |
| Grundy County | 2 | 3.66% | 54,436 | 52,516 |
| McHenry County | 3 | 1.74% | 315,284 | 309,882 |
| Will County | 4 | 1.33% | 706,092 | 696,826 |
| Piatt County | 5 | 0.67% | 16,792 | 16,681 |
| Brown County | 6 | 0.37% | 6,250 | 6,227 |
| Boone County | 7 | 0.24% | 53,470 | 53,343 |
| Woodford County | 8 | 0.23% | 38,501 | 38,413 |
| DeKalb County | 9 | 0.15% | 100,418 | 100,269 |
| Kane County | 10 | 0.09% | 516,668 | 516,181 |
| Marshall County | 11 | -0.09% | 11,707 | 11,717 |
| Williamson County | 12 | -0.13% | 67,030 | 67,114 |
| Monroe County | 13 | -0.24% | 34,869 | 34,953 |
| Champaign County | 14 | -0.52% | 204,594 | 205,654 |
| De Witt County | 15 | -0.62% | 15,365 | 15,461 |
| LaSalle County | 16 | -0.77% | 108,565 | 109,410 |
| McLean County | 17 | -0.86% | 169,487 | 170,949 |
| Effingham County | 18 | -0.89% | 34,393 | 34,703 |
| Johnson County | 19 | -0.92% | 13,196 | 13,318 |
| Putnam County | 20 | -0.94% | 5,577 | 5,630 |
| Lake County | 21 | -0.95% | 706,482 | 713,281 |
| Douglas County | 22 | -1.07% | 19,553 | 19,764 |
| Ogle County | 23 | -1.14% | 51,105 | 51,692 |
| Clinton County | 24 | -1.14% | 36,481 | 36,901 |
| DuPage County | 25 | -1.18% | 920,311 | 931,317 |
| Tazewell County | 26 | -1.28% | 129,403 | 131,086 |
| Kankakee County | 27 | -1.34% | 105,824 | 107,264 |
| Moultrie County | 28 | -1.42% | 14,276 | 14,481 |
| Mercer County | 29 | -1.52% | 15,395 | 15,632 |
| McDonough County | 30 | -1.52% | 26,689 | 27,100 |
| Carroll County | 31 | -1.61% | 15,420 | 15,672 |
| Jasper County | 32 | -1.66% | 9,088 | 9,241 |
| Logan County | 33 | -1.7% | 27,438 | 27,912 |
| Madison County | 34 | -1.71% | 261,112 | 265,643 |
| Winnebago County | 35 | -1.84% | 279,682 | 284,916 |
| Franklin County | 36 | -1.89% | 36,984 | 37,697 |
| Perry County | 37 | -1.92% | 20,485 | 20,886 |
| Bureau County | 38 | -1.98% | 32,533 | 33,191 |
| Sangamon County | 39 | -2.03% | 192,171 | 196,144 |
| Ford County | 40 | -2.04% | 13,238 | 13,513 |
| Henry County | 41 | -2.06% | 48,152 | 49,167 |
| Whiteside County | 42 | -2.13% | 54,384 | 55,565 |
| Adams County | 43 | -2.14% | 64,211 | 65,614 |
| Hardin County | 44 | -2.17% | 3,563 | 3,642 |
| Jo Daviess County | 45 | -2.17% | 21,578 | 22,057 |
| Randolph County | 46 | -2.32% | 29,393 | 30,090 |
| Macoupin County | 47 | -2.34% | 43,808 | 44,856 |
| Jefferson County | 48 | -2.36% | 36,122 | 36,995 |
| Livingston County | 49 | -2.42% | 34,926 | 35,792 |
| Bond County | 50 | -2.43% | 16,278 | 16,684 |
| Fayette County | 51 | -2.5% | 20,952 | 21,490 |
| Schuyler County | 52 | -2.55% | 6,693 | 6,868 |
| Cumberland County | 53 | -2.55% | 10,167 | 10,433 |
| Rock Island County | 54 | -2.61% | 140,590 | 144,360 |
| Washington County | 55 | -2.63% | 13,362 | 13,723 |
| Peoria County | 56 | -2.64% | 176,539 | 181,319 |
| Jackson County | 57 | -2.69% | 51,469 | 52,891 |
| Coles County | 58 | -2.72% | 45,488 | 46,760 |
| Richland County | 59 | -2.76% | 15,354 | 15,790 |
| Jersey County | 60 | -2.81% | 20,891 | 21,494 |
| Hancock County | 61 | -2.81% | 17,050 | 17,543 |
| Hamilton County | 62 | -2.85% | 7,771 | 7,999 |
| Lee County | 63 | -2.87% | 33,086 | 34,062 |
| Shelby County | 64 | -3.04% | 20,284 | 20,919 |
| Morgan County | 65 | -3.11% | 31,824 | 32,845 |
| Christian County | 66 | -3.11% | 32,904 | 33,960 |
| Clark County | 67 | -3.14% | 14,944 | 15,429 |
| St. Clair County | 68 | -3.24% | 248,524 | 256,844 |
| Clay County | 69 | -3.24% | 12,843 | 13,273 |
| Wayne County | 70 | -3.28% | 15,597 | 16,126 |
| Edgar County | 71 | -3.3% | 16,232 | 16,786 |
| Pope County | 72 | -3.33% | 3,629 | 3,754 |
| Montgomery County | 73 | -3.6% | 27,187 | 28,202 |
| Knox County | 74 | -3.61% | 47,993 | 49,790 |
| Calhoun County | 75 | -3.68% | 4,245 | 4,407 |
| Cass County | 76 | -3.69% | 12,512 | 12,991 |
| Marion County | 77 | -3.83% | 36,209 | 37,652 |
| Crawford County | 78 | -3.97% | 17,910 | 18,651 |
| Lawrence County | 79 | -4% | 14,657 | 15,268 |
| Pike County | 80 | -4.18% | 14,028 | 14,640 |
| Union County | 81 | -4.23% | 16,457 | 17,184 |
| Cook County | 82 | -4.28% | 5,038,084 | 5,263,608 |
| Vermilion County | 83 | -4.4% | 70,750 | 74,009 |
| Macon County | 84 | -4.48% | 99,107 | 103,752 |
| Menard County | 85 | -4.5% | 11,718 | 12,270 |
| Greene County | 86 | -4.92% | 11,349 | 11,936 |
| Saline County | 87 | -4.96% | 22,515 | 23,690 |
| Wabash County | 88 | -5.05% | 10,746 | 11,318 |
| Stephenson County | 89 | -5.17% | 42,203 | 44,503 |
| Fulton County | 90 | -5.18% | 31,753 | 33,487 |
| Massac County | 91 | -5.69% | 13,329 | 14,133 |
| White County | 92 | -5.76% | 13,013 | 13,808 |
| Iroquois County | 93 | -5.79% | 25,422 | 26,984 |
| Warren County | 94 | -5.86% | 15,835 | 16,821 |
| Pulaski County | 95 | -6.34% | 4,833 | 5,160 |
| Henderson County | 96 | -6.42% | 5,928 | 6,335 |
| Stark County | 97 | -6.58% | 5,028 | 5,382 |
| Edwards County | 98 | -6.77% | 5,788 | 6,208 |
| Mason County | 99 | -6.79% | 12,171 | 13,058 |
| Scott County | 100 | -7.14% | 4,588 | 4,941 |
| Gallatin County | 101 | -11.57% | 4,364 | 4,935 |
| Alexander County | 102 | -14.82% | 4,403 | 5,169 |
Illinois's counties show varied growth patterns from 2020 to 2025. Kendall County leads with 9.51% annualized growth, followed by Grundy County (3.66%) and McHenry County (1.74%). The largest counties are Cook County (5,038,084), DuPage County (920,311), and Lake County (706,482). Of the 102 counties in Illinois, just 10 are growing while 92 have experienced population decline. Overall, counties averaged -2.67% annualized growth, with total population growing by -2.39% annually.