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.

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, 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 then-incumbent 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.

Thornton is located at 39°54′11″N 104°57′16″W / 39.90306°N 104.95444°W / 39.90306; -104.95444 (39.903043, -104.954406).

At the 2020 United States Census, the town had a total area of 24,286 acres (98.282 km2) including 1,295 acres (5.239 km2) of water.

As of the 2010 census, there were 118,772 people, 41,359 households, and 30,254 families living in the city. The population density was 3,409.1 inhabitants per square mile (1,316.3/km2). There were 43,230 housing units at an average density of 1,240.8 per square mile (479.1/km2). The racial makeup of the city was 77.4% White, 4.4% Asian, 1.8% African American, 1.1% American Indian, 0.1% Pacific Islander, 11.4% from other races, and 3.8% from two or more races. Hispanics and Latinos of any race were 31.7% of the population.

There were 41,359 households, out of which 43.9% had children under the age of 18 living with them, 54.7% were married couples living together, 6.2% had a male householder with no wife present, 12.2% had a female householder with no husband present, and 26.9% were non-families. 20.1% of all households were made up of individuals, and 4.2% had someone living alone who was 65 years of age or older. The average household size was 2.86, and the average family size was 3.32.

The distribution of the population by age was 29.5% under the age of 18, 9.1% from 18 to 24, 32.6% from 25 to 44, 22.3% from 45 to 64, and 6.5% who were 65 years of age or older. The median age was 32.0 years. The gender makeup of the city was 49.5% male and 50.5% female.

The median income for a household in the city was $65,578, and the median income for a family was $74,233. Males had a median income of $49,154 versus $39,596 for females. The city's per capita income was $26,100. About 7.8% of families and 9.9% of the population were below the poverty line, including 14.1% of those under age 18 and 7.4% of those age 65 or over.

Thornton has 81 city parks and nearly 2,000 acres (8.1 km2) of parks and open space. There are over 80 miles (130 km) of trails throughout the city. Recreational facilities include the Margaret Carpenter Recreation Center and the Thornton Community Center. Golf courses include the Thorncreek Golf Course and Todd Creek Golf.

Six highways run through the city of Thornton:

The Regional Transportation District provides bus and commuter rail service to Thornton. Commuter rail service to the city began on September 21, 2020, on the N Line. There are three commuter rail stations within the city, Original Thornton/88th station, Thornton Crossroads/104th station, and Eastlake/124th station, which currently serves as the northern terminus of the N Line.

Thornton is served primarily by four school districts: Adams County School District 12, Adams County School District 14 Mapleton Public Schools, and Brighton School District 27J. These include: Eight high schools (Thornton High School and Horizon High School {Both Adams 12}, Riverdale Ridge High School {Brighton 27J}, and five of Mapleton's small-by-design high schools), eleven middle schools (Five in Adams 12, one in Brighton 27J, five in Mapleton) and twenty elementary schools (Twelve in Adams 12, two in Brighton 27J, and six in Mapleton).

There are also several charter schools in or near Thornton, including Stargate School, Colorado Virtual Academy, New America School, and Westgate Community Charter School.

Lord of Life Lutheran School is a Christian Pre-K–8 school of the Wisconsin Evangelical Lutheran Synod in Thornton.

Thornton has several libraries and is served by the Rangeview Library District, also known as Anythink Libraries.

Thornton has several shopping areas, such as Larkridge Mall, Thornton Town Center, and Thorncreek Crossing Shopping Center. Larkridge is home to national anchor tenants, big box retailers and restaurants, and has a pedestrian village. DaveCo Liquor Store is the largest liquor store in the world.