The City of Thornton is a home rule municipality located in Adams County and Weld County, Colorado, United States. As of 2005, the city is estimated to have a total population of 105,182. Thornton is the sixth most populous city in the State of Colorado and the 225th most populous city in the United States. Thornton apartments for rent comparison page shows most up to date rent information on Thornton apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.