20 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marshall township | 1 | 4,430 | 4,561 | -0.47% | 140 | 31.71 | Township |
| Marshall | 2 | 3,831 | 3,951 | -0.62% | 1,044 | 3.67 | City |
| Casey township | 3 | 3,354 | 3,465 | -0.47% | 93 | 36.22 | Township |
| Casey | 4 | 2,313 | 2,405 | -0.77% | 882 | 2.62 | City |
| Wabash township | 5 | 2,033 | 2,103 | -0.49% | 28 | 72.97 | Township |
| Martinsville township | 6 | 1,483 | 1,532 | -0.47% | 36 | 41.64 | Township |
| Martinsville | 7 | 1,068 | 1,113 | -0.84% | 528 | 2.02 | City |
| Westfield township | 8 | 833 | 851 | -0.12% | 24 | 34.72 | Township |
| Westfield township | 9 | 619 | 647 | -0.8% | 35 | 17.76 | Township |
| Westfield | 10 | 513 | 535 | -0.77% | 525 | 0.98 | Village |
| York township | 11 | 484 | 498 | -0.41% | 15 | 31.27 | Township |
| Anderson township | 12 | 415 | 430 | -0.48% | 13 | 32.31 | Township |
| Johnson township | 13 | 365 | 374 | -0.27% | 10 | 36.47 | Township |
| Dolson township | 14 | 349 | 360 | -0.29% | 10 | 34.54 | Township |
| Melrose township | 15 | 328 | 339 | -0.61% | 10 | 32.88 | Township |
| Darwin township | 16 | 324 | 335 | -0.61% | 12 | 27.16 | Township |
| Orange township | 17 | 249 | 256 | -0.4% | 7 | 37.12 | Township |
| West Union | 18 | 174 | 241 | -4.4% | 96 | 1.81 | CDP |
| Parker township | 19 | 158 | 164 | -0.63% | 4 | 35.87 | Township |
| Douglas township | 20 | 127 | 130 | 0% | 7 | 17.66 | Township |
Marshall township is the largest city in Clark County, with a population of 4,430.
Douglas township is the smallest city in Clark County.
There are 20 cities in Clark County, Illinois.