[4] According to the U.S. Census Bureau, the county has a total area of 1,134 square miles (2,940 km2), of which 1,134 sq mi (2,940 km2) are land and 0.8 sq mi (2.1 km2) (0.07%) is covered by water.

[5] As of the 2020 United States census, there were 18,385 people, 4,673 households, and 3,306 families residing in the county.

As of the census[12] of 2000, 16,252 people, 4,743 households, and 3,642 families resided in the county.

About 20.60% of all households was made up of individuals, and 9.30% had someone living alone who was 65 years of age or older.

Frio County has generally leaned Democratic at the presidential level, although it has sometimes voted Republican, including the elections of 2004 and 2020.