Fillmore is a city and the county seat of Millard County, Utah, United States. The population was 2,592 at the 2020 United States Census. …Fillmore is a city and the county seat of Millard County, Utah, United States. The population was 2,592 at the 2020 United States Census. It is named for the thirteenth U.S. President Millard Fillmore, who was in office when Millard County was created by the Utah Territorial legislature.