In 2019, the most highly populated county within the state of Illinois is Cook County, home to the city of Chicago. With a population of 5,180,493, Cook County has seen a decrease in the total population of 0.36% since the last U.S. census was taken in 2010. Following Cook County, other large counties in Illinois are DuPage County (928,589), Lake County (700,832), Will County (692,310), and Kane County (534,216). Kane County reported the most significant population growth, increasing its population by 3.50%.
The county with the lowest population in the state of Illinois is Hardin County, which has a population of 3,910. Since the last census in 2010, Hardin County’s population has seen a decrease of 9.35%. Following behind Hardin County, other small counties in Illinois include Pope County (4,212), Calhoun County (4,802), and Scott County (4,926). Illinois has 72 counties with a population of less than 50,000; all but one (Monroe County) experienced a decline in population. Of the 15 counties with fewer than 10,000 residents, Alexander County experienced the most dramatic decrease with a growth rate of -26.15%.
Kendall County is the fastest-growing county in Illinois with a population growth rate of 10.87% to 127,915 residents. Following Kendall County, Champagne County (4.19%), Monroe County (4.05%), and Kane County (3.5%) also experienced an increase in residents. Only five other counties in Illinois had any increase in population since 2010. The county with the lowest growth rate in Illinois is Alexander County, which has seen a population decrease of 26.15%.