Costilla County was of the original 17 counties
created by the Colorado legislature on November 1,
1861. It was named after the Costilla River in the
area. Although San Miguel was originally designated
the county seat, county government was moved to San
Luis in 1863.
The county's original boundaries had the county extend over much of south-central Colorado. Much of the northern portion became part of Saguache County in 1866, and the western portions were folded into Hinsdale and Rio Grande counties in 1874. Costilla County arrived at its modern boundaries in 1913 when Alamosa County was created from its northwest portions.

