The area was originally titled Mesquite Flat when the town applied for its own post office. The application was rejected, and resubmitted as Flat. The post office was successfully opened in 1897; its ZIP code is 76526.
In 1914 Flat had a thriving population of 100, with 3 general stores and a cotton gin. By the mid-1920s, population estimates fell to 25. In the late 1930s, the population had rebounded to 125.
With the establishment of Fort Hood in 1942, much of the towns cropland was converted into a military base. In 1960, the population was 200. The Flat school merged with Gatesville ISD in 1963. In 1970, the population was 210.