Fillmore is a small city in Ventura County, California, United States in the Santa Clara River Valley. In an agricultural area with rich, fertile soil, Fillmore has a historic downtown that was established when the Southern Pacific built the railroad through the valley in 1887. The rail line also provided a name for the town: J. A. Fillmore was a general superintendent for the company's Pacific system. The population was 15,002 at the 2010 census, up from 13,643 at the 2000 census.