The area was originally titled "Mesquite Flat" when the town applied for its own post office.[3] The application was rejected, and resubmitted as "Flat". The post office was successfully opened in 1897;[3] its ZIP code is 76526.[4]
In 1914, Flat had a thriving population of 100, with three 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 town's 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. As of 2000, the population was 861. From 2000-2020, the population decreased, to a total of 157.[2]
Flat CDP, Texas – Racial and ethnic composition Note: the US Census treats Hispanic/Latino as an ethnic category. This table excludes Latinos from the racial categories and assigns them to a separate category. Hispanics/Latinos may be of any race.