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.