The Clovis are the earliest known people to inhabit the territory before Mills County, though recent discoveries indicate that there were people living in the area as far back as 15,000 to 20,000 years. More recently, the Tonkawa occupied it, and there are numerous vestiges from their campsites that remain across the county, including cooking middens. Thought to be the first white man to explore pre-Mills County, Pedro Vial visited in 1786 and 1789 while traveling between San Antonio and Santa Fe. Captain Henry S. Brown, believed to be the first white visitor, led a group to the area in 1825 to recover stolen stock. Mills County was once a part of two Mexican municipalities, Milam (originally Viesca Municipality) and Bastrop (originally Mina Municipality).
