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 (16 km) north of the Colorado State Capitol in Denver and is a part of the Denver–Aurora–Lakewood, CO Metropolitan Statistical Area.