Thornton is on State Highway 14 nineteen miles southwest of Mexia in southwestern Limestone County. Settlement at the site began in 1868. The Houston and Texas Central Railway ran a line from Hearne to Groesbeck through the area in 1870, and Cain Hogan settled on Steele Creek just west of the present townsite. The community became a stop on the railroad and was named for John E. Thornton, another early settler.
In 1873 a Thornton post office opened with William L. Wright as postmaster. Soon cotton gins were set up nearby, and Wilson Brothers, a mercantile business, located in Thornton. Ceramics were manufactured from the fine, white clay called kaolin found in that locality. Agriculture was the mainstay of the area. Thornton Institute, an early college, was founded in 1877 by Edward Coke Chambers. He was also placed in charge of the local grade school, which he turned into a preparatory school. He quit both schools and left Thornton in the late 1880s, and the institute was sold to Henry P. Davis. After enrollment dropped, the institute became Thornton School. The Thornton Masonic lodge, No. 486, was chartered with thirty-two members in 1878. Other organizations followed, including the International Organization of Odd Fellows, the Knights of Pythias, the Woodmen of the World, and the Order of the Eastern Star.
By 1880 Thornton had a population of 200, thirteen businesses, and three churches. In 1890 it had a population of 466, steam gristmills, and a local newspaper called the Herald, which merged with the Groesbeck Journal in the 1940s. Later newspapers included the Hustler, the Democratic Eagle, the Topic, and the Press. In 1904 the population was 586, and unsuccessful attempts were made to incorporate. Thornton was incorporated in 1907 for municipal purposes, and W. C. Cayton was elected mayor. During this period the population steadily grew and business continued to expand. In addition to the cotton gins, schools, churches, and steam gristmills, the town soon had eighteen brick stores, a bank, a commission house, a hotel, and electric lights. Between 1910 and 1920 the population count remained at 678. On February 10, 1918, Cyrus F. Smythe landed the first airplane at Thornton, flying in from Houston. By 1926 Thornton had 733 residents, a deep water well, and telephone connections. The total enrollment in the local schools was 259 students. The next year the population reportedly rose to almost 2,000, and another bank was opened in town. By 1931 the population was back down to 739, and thirty-five businesses were located there. Between 1969 and 1989 Thornton had a population between 408 and 498, and the number of businesses was five. In 1990 the population stood at 540. The population was 525 in 2000.