| Rank |
County |
Population (2012) |
| 1 |
Cook County, Illinois
|
5,231,351
|
| 2 |
DuPage County, Illinois
|
927,987
|
| 3 |
Lake County, Illinois
|
702,120
|
| 4 |
Will County, Illinois
|
682,518
|
| 5 |
Kane County, Illinois
|
522,487
|
| 6 |
McHenry County, Illinois
|
308,145
|
| 7 |
Winnebago County, Illinois
|
292,069
|
| 8 |
St Clair County, Illinois
|
268,858
|
| 9 |
Madison County, Illinois
|
267,883
|
| 10 |
Champaign County, Illinois
|
203,276
|
| 11 |
Sangamon County, Illinois
|
199,271
|
| 12 |
Peoria County, Illinois
|
187,254
|
| 13 |
McLean County, Illinois
|
172,281
|
| 14 |
Rock Island County, Illinois
|
147,457
|
| 15 |
Tazewell County, Illinois
|
135,949
|
| 16 |
Kendall County, Illinois
|
118,105
|
| 17 |
Kankakee County, Illinois
|
113,040
|
| 18 |
LaSalle County, Illinois
|
112,973
|
| 19 |
Macon County, Illinois
|
110,122
|
| 20 |
DeKalb County, Illinois
|
104,704
|
| 21 |
Vermilion County, Illinois
|
80,727
|
| 22 |
Adams County, Illinois
|
67,197
|
| 23 |
Williamson County, Illinois
|
66,674
|
| 24 |
Jackson County, Illinois
|
60,071
|
| 25 |
Whiteside County, Illinois
|
57,846
|
| 26 |
Boone County, Illinois
|
53,940
|
| 27 |
Coles County, Illinois
|
53,655
|
| 28 |
Ogle County, Illinois
|
52,848
|
| 29 |
Knox County, Illinois
|
52,247
|
| 30 |
Grundy County, Illinois
|
50,281
|
| 31 |
Henry County, Illinois
|
50,155
|
| 32 |
Macoupin County, Illinois
|
47,231
|
| 33 |
Stephenson County, Illinois
|
46,959
|
| 34 |
Franklin County, Illinois
|
39,407
|
| 35 |
Woodford County, Illinois
|
38,971
|
| 36 |
Marion County, Illinois
|
38,894
|
| 37 |
Jefferson County, Illinois
|
38,720
|
| 38 |
Livingston County, Illinois
|
38,647
|
| 39 |
Clinton County, Illinois
|
38,061
|
| 40 |
Fulton County, Illinois
|
36,651
|
| 41 |
Morgan County, Illinois
|
35,272
|
| 42 |
Lee County, Illinois
|
35,037
|
| 43 |
Christian County, Illinois
|
34,638
|
| 44 |
Effingham County, Illinois
|
34,353
|
| 45 |
Bureau County, Illinois
|
34,323
|
| 46 |
Monroe County, Illinois
|
33,357
|
| 47 |
Randolph County, Illinois
|
32,956
|
| 48 |
McDonough County, Illinois
|
32,537
|
| 49 |
Logan County, Illinois
|
30,013
|
| 50 |
Montgomery County, Illinois
|
29,620
|
| 51 |
Iroquois County, Illinois
|
29,240
|
| 52 |
Saline County, Illinois
|
24,946
|
| 53 |
Jersey County, Illinois
|
22,742
|
| 54 |
Jo Daviess County, Illinois
|
22,549
|
| 55 |
Shelby County, Illinois
|
22,196
|
| 56 |
Perry County, Illinois
|
22,058
|
| 57 |
Fayette County, Illinois
|
22,014
|
| 58 |
Douglas County, Illinois
|
19,853
|
| 59 |
Crawford County, Illinois
|
19,600
|
| 60 |
Hancock County, Illinois
|
18,891
|
| 61 |
Edgar County, Illinois
|
18,191
|
| 62 |
Warren County, Illinois
|
17,731
|
| 63 |
Union County, Illinois
|
17,647
|
| 64 |
Bond County, Illinois
|
17,644
|
| 65 |
Lawrence County, Illinois
|
16,604
|
| 66 |
Wayne County, Illinois
|
16,574
|
| 67 |
Piatt County, Illinois
|
16,504
|
| 68 |
De Witt County, Illinois
|
16,434
|
| 69 |
Pike County, Illinois
|
16,308
|
| 70 |
Mercer County, Illinois
|
16,219
|
| 71 |
Clark County, Illinois
|
16,209
|
| 72 |
Richland County, Illinois
|
16,176
|
| 73 |
Massac County, Illinois
|
15,234
|
| 74 |
Carroll County, Illinois
|
15,011
|
| 75 |
Moultrie County, Illinois
|
14,933
|
| 76 |
Washington County, Illinois
|
14,598
|
| 77 |
White County, Illinois
|
14,568
|
| 78 |
Mason County, Illinois
|
14,327
|
| 79 |
Ford County, Illinois
|
14,008
|
| 80 |
Clay County, Illinois
|
13,766
|
| 81 |
Greene County, Illinois
|
13,576
|
| 82 |
Cass County, Illinois
|
13,338
|
| 83 |
Johnson County, Illinois
|
12,760
|
| 84 |
Menard County, Illinois
|
12,722
|
| 85 |
Marshall County, Illinois
|
12,327
|
| 86 |
Wabash County, Illinois
|
11,727
|
| 87 |
Cumberland County, Illinois
|
10,968
|
| 88 |
Jasper County, Illinois
|
9,614
|
| 89 |
Hamilton County, Illinois
|
8,370
|
| 90 |
Alexander County, Illinois
|
7,748
|
| 91 |
Schuyler County, Illinois
|
7,457
|
| 92 |
Henderson County, Illinois
|
7,043
|
| 93 |
Brown County, Illinois
|
6,914
|
| 94 |
Edwards County, Illinois
|
6,684
|
| 95 |
Pulaski County, Illinois
|
5,998
|
| 96 |
Stark County, Illinois
|
5,946
|
| 97 |
Putnam County, Illinois
|
5,886
|
| 98 |
Gallatin County, Illinois
|
5,430
|
| 99 |
Scott County, Illinois
|
5,290
|
| 100 |
Calhoun County, Illinois
|
5,014
|
| 101 |
Pope County, Illinois
|
4,272
|
| 102 |
Hardin County, Illinois
|
4,258
|