| Name | What | Region | Country | Lat | Long | Elev Ft. | Pop Est |
| Clank | city | Illinois | United States | 37.18833 | -89.325 | 341 | 1069 |
| Clare | city | Illinois | United States | 42.01611 | -88.82917 | 859 | 3843 |
| Claremont | city | Illinois | United States | 38.71944 | -87.97139 | 495 | 932 |
| Claremont Cottages subdivision | city | Illinois | United States | 41.86806 | -87.68472 | 593 | 633855 |
| Clarence | city | Illinois | United States | 40.46389 | -87.97083 | 767 | 813 |
| Clarendon Hills | city | Illinois | United States | 41.7975 | -87.95472 | 748 | 138658 |
| Clarion | city | Illinois | United States | 41.505 | -89.19194 | 744 | 2875 |
| Clark | city | Illinois | United States | 39.25972 | -90.57778 | 429 | 606 |
| Clark Center | city | Illinois | United States | 39.3625 | -87.78194 | 610 | 1013 |
| Clarkdale subdivision | city | Illinois | United States | 41.74278 | -87.71611 | 616 | 311275 |
| Clarke City | city | Illinois | United States | 41.15806 | -88.24278 | 580 | 2365 |
| Clarksburg | city | Illinois | United States | 39.33 | -88.74167 | 610 | 994 |
| Clarksburg IL | city | Illinois | United States | 39.31806 | -88.76444 | 600 | 1042 |
| Clarksdale | city | Illinois | United States | 39.48972 | -89.36778 | 603 | 1627 |
| Clarksville | city | Illinois | United States | 40.65056 | -88.84833 | 698 | 1657 |
| Clarksville | city | Illinois | United States | 39.45167 | -87.79778 | 646 | 712 |
| Clarmin | city | Illinois | United States | 38.22111 | -89.69944 | 498 | 1388 |
| Claus | city | Illinois | United States | 41.36972 | -87.61 | 761 | 6304 |
| Clay City | city | Illinois | United States | 38.68861 | -88.35417 | 446 | 1075 |
| Claypool | city | Illinois | United States | 41.34583 | -88.38722 | 498 | 10120 |
| Claypool Woods | city | Illinois | United States | 41.34417 | -88.4 | 515 | 10120 |
| Clays Prairie | city | Illinois | United States | 39.68694 | -87.56333 | 623 | 1495 |
| Claysville | city | Illinois | United States | 39.86722 | -89.90028 | 600 | 1008 |
| Clayton | city | Illinois | United States | 40.03139 | -90.96056 | 721 | 1032 |
| Claytonville | city | Illinois | United States | 40.56694 | -87.82306 | 656 | 715 |
| Clear Lake | city | Illinois | United States | 40.05056 | -90.30889 | 439 | 789 |
| Clear Lake | city | Illinois | United States | 39.81361 | -89.56861 | 551 | 27004 |
| Clearing | city | Illinois | United States | 41.77806 | -87.76167 | 613 | 245723 |
| Clearing subdivision | city | Illinois | United States | 41.78056 | -87.77333 | 616 | 245723 |
| Cleaverville subdivision | city | Illinois | United States | 41.82694 | -87.60528 | 587 | 387164 |
| Cleburne | city | Illinois | United States | 37.93583 | -89.05361 | 413 | 6275 |
| Clements | city | Illinois | United States | 39.61889 | -90.15083 | 662 | 974 |
| Cleone | city | Illinois | United States | 39.42306 | -87.9075 | 620 | 627 |
| Cleveland | city | Illinois | United States | 41.50611 | -90.31472 | 580 | 7546 |
| Cleveland Subdivision subdivision | city | Illinois | United States | 42.22167 | -89.05972 | 797 | 96995 |
| Cliffdale | city | Illinois | United States | 39.36056 | -90.62639 | 423 | 741 |
| Clifford | city | Illinois | United States | 37.82361 | -89.07722 | 380 | 16767 |
| Clifton | city | Illinois | United States | 40.93528 | -87.93444 | 652 | 1233 |
| Clifton Terrace | city | Illinois | United States | 38.92944 | -90.25556 | 462 | 9734 |
| Clifty Heights | city | Illinois | United States | 37.60056 | -88.94361 | 600 | 1946 |
| Clinton | city | Illinois | United States | 40.15361 | -88.96444 | 715 | 8580 |
| Clintonville | city | Illinois | United States | 42.00056 | -88.27333 | 751 | 90130 |
| Cloverdale | city | Illinois | United States | 40.64028 | -89.51611 | 698 | 32336 |
| Cloverdale | city | Illinois | United States | 41.93944 | -88.12028 | 764 | 136934 |
| Clyde subdivision | city | Illinois | United States | 41.84111 | -87.77306 | 606 | 484701 |
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