U.S. Census Bureau Census Data Shows Illinois First In Population Decline By WNIJ News • Mar 31, 2015 NIU Center for Governmental Studies New U.S. Census data shows Illinois had nearly 10,000 fewer residents in 2014 than the previous year. That’s the largest population decline of any state in the country and the first statewide population dip since the mid-1980s. What Does Slow Population Growth Mean For Illinois? By Mike Moen • Jan 3, 2014 The U.S. Census Bureau says since the last official census in 2010, Illinois has only gained about 52-thousand residents. That puts it in the bottom 10 states when it comes to population change during that time period. No Big Change In Illinois' Population By Mike Moen • Jan 2, 2014 The latest U.S. Census figures reveal slow population growth for Illinois.