Thornton was named in honor of then-incumbent Colorado Governor Dan Thornton, who was known for his Stetson hat, pipe, and cowboy boots.

Thornton consisted solely of farmland until 1953, when Sam Hoffman purchased a lot off Washington Street about seven miles (11 km) north of Denver. The town he laid out was the first fully planned community in Adams County. The Thornton Community Association (TCA) was formed in 1954 to help guide the new community's development, and by the end of 1955, Thornton had 5,500 residents in over 1,200 homes.

When F&S Construction Company opened Thornton's first model homes for tours, they hired Hollywood legend Jane Russell to decorate the model homes and greet people at the new housing development. Thousands of people turned out. Thornton's Russell Boulevard is named after the actress.

On August 18, 1955, voters rejected incorporation with 548 votes for and 620 votes against. Less than a year later, on May 26, 1956, voters approved the measure and Thornton was incorporated as a Colorado city. At the time, Thornton had a population of 8,640 and was just one square mile in size.

Riverdale Road, a winding 11-mile road that connects Brighton and Thornton, has a history of alleged hauntings and urban legends--from the existence of the literal "Gates of Hell" to the story of a man who lost his mind and burned his entire mansion while his family slept inside. The road is also home to so-called "Jogger's Hill," where the ghost of a murdered jogger is rumored to follow cars and tap on their sides.

In 2009, Daveco Liquors in Thornton, Colorado, was certified by Guinness World Records as the "World's Largest Liquor Store." The shop covers an area of 100,073.1 square feet (almost 2.3 acres), and includes a huge beer cooler and a wine tasting room.

Thornton's first shopping center opened on Washington Street in May 1955. It contained stores such as Woolworths and Millers grocery store. Originally called Hoffman Heights Shopping Center, the name soon changed to the Thornton Shopping Center.

In 1965, Thornton selected a city seal by holding a contest that was open to all Thornton public school children. The winner was a 17-year-old Mapleton High School student, who proposed a city seal with the motto "City of Planned Progress".

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.

For the 1996 celebration of the city's 40th anniversary, a community festival called Thorntonfest was founded. The festival, which has been held every year since, encourages Thornton residents to come out for daylong festivities to celebrate the community and get to know their neighbors.