| City/Town Name | Population |
| Chicago | 2,709,534 |
| Elgin | 112,653 |
| Cicero | 82,330 |
| Arlington Heights | 75,482 |
| Evanston | 74,587 |
| Schaumburg | 74,194 |
| Palatine | 68,407 |
| Skokie | 63,821 |
| Orland Park | 58,749 |
| Des Plaines | 58,673 |
| Tinley Park | 56,505 |
| Oak Lawn | 55,936 |
| Berwyn | 55,407 |
| Mount Prospect | 54,604 |
| Oak Park | 52,233 |
| Hoffman Estates | 50,841 |
| Glenview | 47,416 |
| Elmhurst | 46,463 |
| Bartlett | 41,120 |
| Buffalo Grove | 41,062 |
| Streamwood | 39,809 |
| Wheeling | 38,499 |
| Hanover Park | 37,984 |
| Park Ridge | 37,457 |
| Calumet City | 36,551 |
| Woodridge | 33,455 |
| Northbrook | 33,343 |
| Elk Grove Village | 32,942 |
| Chicago Heights | 29,856 |
| Niles | 29,451 |
| Burbank | 28,729 |
| Lansing | 27,904 |
| Oak Forest | 27,617 |
| Wilmette | 27,247 |
| Melrose Park | 25,605 |
| Elmwood Park | 24,468 |
| Harvey | 24,386 |
| Maywood | 23,578 |
| Rolling Meadows | 23,219 |
| Morton Grove | 23,089 |
| Roselle | 22,754 |
| Dolton | 22,737 |
| Blue Island | 22,611 |
| South Holland | 21,677 |
| Park Forest | 21,563 |
| Evergreen Park | 19,479 |
| Matteson | 19,385 |
| Homewood | 19,112 |
| Alsip | 19,022 |
| Deerfield | 19,006 |
| Frankfort | 18,999 |
| Bellwood | 18,996 |
| Brookfield | 18,735 |
| Bensenville | 18,281 |
| Franklin Park | 17,956 |
| Hinsdale | 17,710 |
| Palos Hills | 17,318 |
| Lemont | 17,023 |
| Country Club Hills | 16,758 |
| Westchester | 16,440 |
| Prospect Heights | 16,137 |
| Bridgeview | 16,124 |
| La Grange | 15,545 |
| Norridge | 14,703 |
| Midlothian | 14,595 |
| Chicago Ridge | 14,153 |
| Forest Park | 13,927 |
| Hickory Hills | 13,816 |
| Hazel Crest | 13,549 |
| Richton Park | 13,504 |
| La Grange Park | 13,395 |
| Western Springs | 13,272 |
| Justice | 12,800 |
| Riverdale | 12,793 |
| Markham | 12,493 |
| Palos Heights | 12,480 |
| Lincolnwood | 12,434 |
| Winnetka | 12,428 |
| Northlake | 12,290 |
| Schiller Park | 11,621 |
| Summit | 11,260 |
| River Forest | 10,970 |
| Crestwood | 10,831 |
| Burr Ridge | 10,763 |
| Worth | 10,633 |
| Lyons | 10,470 |
| Sauk Village | 10,423 |
| Barrington | 10,290 |
| River Grove | 10,076 |
| Flossmoor | 9,472 |
| Steger | 9,366 |
| Lynwood | 9,279 |
| Glencoe | 8,888 |
| Riverside | 8,759 |
| Glenwood | 8,657 |
| Harwood Heights | 8,487 |
| Calumet Park | 8,208 |
| Crete | 8,118 |
| Hillside | 8,080 |
| Oak Brook | 8,075 |
| Broadview | 7,755 |
| Inverness | 7,633 |
| Orland Hills | 7,136 |
| University Park | 6,947 |
| Stickney | 6,679 |
| North Riverside | 6,646 |
| Posen | 6,149 |
| Countryside | 5,961 |
| Willow Springs | 5,604 |
| Northfield | 5,534 |
| Berkeley | 5,116 |
| Robbins | 5,037 |
| South Barrington | 4,923 |
| Stone Park | 4,894 |
| Palos Park | 4,842 |
| Olympia Fields | 4,821 |
| Hometown | 4,272 |
| Rosemont | 4,255 |
| Burnham | 4,158 |
| South Chicago Heights | 4,076 |
| Barrington Hills | 4,070 |
| Deer Park | 3,884 |
| Indian Head Park | 3,776 |
| Dixmoor | 3,604 |
| East Dundee | 3,181 |
| Ford Heights | 2,736 |
| Thornton | 2,545 |
| Kenilworth | 2,460 |
| Merrionette Park | 2,050 |
| Phoenix | 1,884 |
| Hodgkins | 1,746 |
| East Hazel Crest | 1,586 |
| Forest View | 943 |
| Bedford Park | 614 |
| Golf | 481 |
| McCook | 279 |
| District 1 | Brandon Johnson |
| District 2 | Dennis Deer |
| District 3 | Bill Lowry |
| District 4 | Stanley Moore |
| District 5 | Deborah Sims |
| District 6 | Donna Miller |
| District 7 | Alma E. Anaya |
| District 8 | Luis Arroyo Jr. |
| District 9 | Peter N. Silvestri |
| District 10 | Bridget Gainer |
| District 11 | John P. Daley |
| District 12 | Bridget Degnen |
| District 13 | Larry Suffredin |
| District 14 | Scott R. Britton |
| District 15 | Kevin B. Morrison |
| District 16 | Frank J. Aguilar |
| District 17 | Sean M. Morrison |
The county has more Democratic Party members than any other Illinois county and it is one of the most Democratic counties in the United States.[29] Since 1932, the majority of its voters have only supported a Republican candidate in a Presidential election three times, all during national Republican landslides–Dwight Eisenhower over native son Adlai Stevenson II in 1952 and 1956, and Richard Nixon over George McGovern in 1972. Since then, the closest a Republican has come to carrying the county was in 1984, when Ronald Reagan won 48.4 percent of the county's vote. In 2020, 74 percent of the county voted for Joe Biden and 24 percent voted for Donald Trump.
In 1936, with Franklin D. Roosevelt receiving 1,253,164 votes in the county, Cook County became the first county in American history where a candidate received one million votes.
The Cook County Democratic Party represents Democratic voters in 50 wards in the city of Chicago and 30 suburban townships of Cook County. The organization has dominated County, city, and state politics since the 1930s. The last Republican mayor of Chicago was William Hale "Big Bill" Thompson, who left office in 1931 with a record of corruption. The most successful Republican candidate for mayor since then was Bernard Epton, who in 1983 came within 3.3 percentage points of defeating Democrat Harold Washington.[30] The county's Republican Party organization is the Cook County Republican Party.
The last Republican governor to carry the county was Jim Edgar in his 1994 landslide. The last Republican senator to do so was Charles H. Percy in 1978.
