The county seat and largest city is Twin Falls.

Even after Idaho became a state in 1890, the Twin Falls area was very sparsely populated and without significant settlements.

That changed after the city of Twin Falls was founded in 1904,[3] and Milner Dam was completed in 1905.

In response to local government needs for these new communities, which were as much as 80 miles (130 km) away from Cassia County's seat at Albion, Twin Falls County was created by the Idaho Legislature on February 21, 1907.

[5] As of the 2010 census, there were 77,230 people, 28,760 households, and 19,954 families living in the county.

[14] As of the 2000 census, there were 64,284 people, 23,853 households, and 16,959 families living in the county.

23.60% of all households were made up of individuals, and 10.40% had someone living alone who was 65 years of age or older.

Twin Falls County leans heavily Republican; it has not voted for a Democratic presidential nominee since 1936, and even in the Democratic landslide of 1964 Barry Goldwater still carried Twin Falls by 20 percentage points.

School districts include:[16] The county is in the catchment area, and the taxation zone, for College of Southern Idaho.