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.

Adams County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 519,572. The county seat is Brighton, and the largest city is Thornton. The county is named for Alva Adams, an early Governor of the State of Colorado in 1887–1889. Adams County is part of the Denver–Aurora–Lakewood, CO Metropolitan Statistical Area.