[1] The county seat is San Luis,[2] the oldest continuously occupied town in Colorado.

On July 8, 1694, Spanish Conquistador Don Diego de Vargas and his army, two weeks before the Battle of Astialakwa, reached Costilla County.

The county made up the major part of the Sangre de Cristo Land Grant awarded by the government of New Mexico to the Carlos Beaubien family in 1843.

Hispanic settlers from Taos, New Mexico, officially established San Luis on April 9, 1851.

Costilla County tends to favor the Democratic candidate in Presidential elections.

The last Republican to carry the county was Calvin Coolidge in 1924,[13] and the last to gain an absolute majority William Howard Taft in 1912 – an era when most votes in these high valley counties were done for the voters by political machines.

In recent years, however, Republicans have gained ground in Costilla County and other areas of southern Colorado, with Republican candidate Donald Trump breaking 40% of the vote while holding Democratic candidate Kamala Harris below 60% of the vote, both for the first time since 1972.

It is part of Colorado's 3rd congressional district, which has a Cook Partisan Voting Index of R+7 and is represented by Republican Jeff Hurd.

In the Colorado Senate, it is in District 6 and is represented by Republican Cleave Simpson.