Thornton is a home rule municipality located in Adams and Weld counties, Colorado, United States. The city population was 141,867, all in Adams County, at the 2020 United States Census, an increase of+19.44% since the 2010 United States Census. Thornton is the sixth-most-populous city in Colorado and the 191st-most-populous city in the United States. Thornton is 10 miles north of the Colorado State Capitol in Denver and is a part of the Denver–Aurora–Lakewood, CO Metropolitan Statistical Area.