CHICAGO—A larger-than-expected exodus over the past 10 years reduced the population of Chicago to a level not seen in nearly a century.

The U.S. Census Bureau reported Tuesday that during the decade ended in 2010, Chicago's population fell 6.9% to 2,695,598 people, fewer than the 2.7 million reported back in 1920.

After peaking at 3.62 million...