20 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marshall | 1 | 4,414 | 4,558 | -0.94% | 139 | 32 | Township |
| Marshall | 2 | 3,843 | 3,951 | -0.57% | 1,048 | 4 | City |
| Casey | 3 | 3,375 | 3,487 | -0.91% | 93 | 36 | Township |
| Casey | 4 | 2,362 | 2,432 | -0.59% | 901 | 3 | City |
| Wabash | 5 | 2,029 | 2,103 | -0.93% | 28 | 73 | Township |
| Martinsville | 6 | 1,475 | 1,527 | -0.87% | 35 | 42 | Township |
| Martinsville | 7 | 1,075 | 1,113 | -0.74% | 532 | 2 | City |
| Westfield | 8 | 815 | 843 | -0.73% | 23 | 35 | Township |
| Westfield | 9 | 626 | 649 | -0.79% | 35 | 18 | Township |
| Westfield | 10 | 516 | 535 | -0.77% | 528 | 1 | Village |
| York | 11 | 482 | 498 | -0.82% | 15 | 31 | Township |
| Anderson | 12 | 416 | 430 | -0.72% | 13 | 32 | Township |
| Johnson | 13 | 358 | 373 | -1.38% | 10 | 36 | Township |
| Dolson | 14 | 346 | 360 | -1.42% | 10 | 35 | Township |
| Melrose | 15 | 329 | 339 | -0.9% | 10 | 33 | Township |
| Darwin | 16 | 322 | 334 | -0.92% | 12 | 27 | Township |
| Orange | 17 | 255 | 262 | -0.78% | 7 | 37 | Township |
| West Union | 18 | 174 | 241 | -4.4% | 96 | 2 | CDP |
| Parker | 19 | 158 | 164 | -1.25% | 4 | 36 | Township |
| Douglas | 20 | 122 | 130 | -2.4% | 7 | 18 | Township |
Marshall is the largest city in Clark County, with a population of 4,414.
Douglas is the smallest city in Clark County, with a population of 122.
There are 20 cities in Clark County, Illinois.