#43. Boone County
- 2010 to 2020 population change: -717
--- #2,344 among all counties nationwide
- 2010 to 2020 percent population change: -1.3%
--- #21 among counties in Illinois, #1,710 among all counties nationwide
- 2020 population: 53,448
--- #25 largest county in Illinois, #943 largest county nationwide
#9. McHenry County
- 2010 to 2020 population change: +1,469
--- #912 among all counties nationwide
- 2010 to 2020 percent population change: +0.5%
--- #14 among counties in Illinois, #1,417 among all counties nationwide
- 2020 population: 310,229
--- #6 largest county in Illinois, #230 largest county in US
#5. Lake County
- 2010 to 2020 population change: +10,880
--- #407 among all counties nationwide
- 2010 to 2020 percent population change: +1.5%
--- #10 among counties in Illinois, #1,247 among all counties nationwide
- 2020 population: 714,342
--- #3 largest county in Illinois, #97 largest county nationwide
#4. DuPage County
- 2010 to 2020 population change: +15,953
--- #336 among all counties nationwide
- 2010 to 2020 percent population change: +1.7%
--- #8 among counties in Illinois, #1,221 among all counties nationwide
- 2020 population: 932,877
--- #2 la
#3. Kendall County
- 2010 to 2020 population change: +17,133
--- #320 among all counties nationwide
- 2010 to 2020 percent population change: +14.9%
--- #1 among counties in Illinois, #229 among all counties nationwide
- 2020 population: 131,869
--- #15 largest county in Illinois, #489 largest county nationwide
#2. Will County
- 2010 to 2020 population change: +18,795
--- #305 among all counties nationwide
- 2010 to 2020 percent population change: +2.8%
--- #5 among counties in Illinois, #1,087 among all counties nationwide
- 2020 population: 696,355
--- #4 largest county in Illinois, #98 largest county nationwide
#1. Cook County
- 2010 to 2020 population change: +80,866
--- #64 among all counties nationwide
- 2010 to 2020 percent population change: +1.6%
--- #9 among counties in Illinois, #1,244 among all counties nationwide
- 2020 population: 5,275,541
--- #1 largest county in Illinois, #2 largest county nationwide
