As of the 2010 census, the population of Allen Parish was 25,764. The parish seat is Oberlin. Allen Parish is in southwestern Louisiana, southwest of Alexandria.

Allen Parish is named for former Confederate States Army general and Governor of Louisiana Henry Watkins Allen. It was separated in 1912 from the larger Calcasieu Parish to the southwest.

On September 27, 2008, the Allen Parish Tourist Commission opened Leatherwood Museum in Oakdale in a two-story house which served during the early 20th century as a hospital where women waited on the second-floor balcony to deliver their babies.

The museum focuses on the history of agriculture and timber.

Residents are zoned to Allen Parish Schools.