Thornton consisted solely of farmland until 1953 when Sam Hoffman purchased a lot off Washington Street about seven miles north of Denver. The town he laid out was the first fully planned community in Adams County, and the first to offer full municipal services from a single tax levy, including recreation services and free trash pickup. Thornton was named in honor of Former Colorado Governor Dan Thornton.

The Thornton Community Association (TCA) was formed in 1954 to help guide the new community’s development. By the end of 1955, Thornton had 5,500 residents in over 1,200 homes. The TCA was instrumental in Thornton’s 1956 incorporation as a city. Oyer G. Leary was elected the first mayor.