In 1986, McClarnon graduated from Omaha Central High School, where he said drama Teacher Peggy Stommes was a big influence.
McClarnon got his start in a local production of Jesus Christ Superstar at the Chanticleer Theater in Council Bluffs, Iowa. He connected with John Jackson, a local Omaha casting Director who later became known for his work with Director Alexander Payne, which led to local work, until McClarnon moved to Los Angeles in the early 1990s.
McClarnon was in the Into the West (2005) miniseries for TNT and played Running Fox. He starred in the 2009 film Not Forgotten and comedy film Repo Chick. In 2013, he was in the horror movie Resolution. He had a recurring role as Bodaway Macawi in Ringer. He also played Mike Parker in the first four episodes of the SundanceTV series The Red Road.
From 2012 to 2017, McClarnon portrayed the Cheyenne reservation Chief of Indian Tribal Police, Mathias, in Longmire, originally an A&E and later a Netflix television series.
In 2017, McClarnon appeared as Toshaway in AMC Network's The Son, which is based on the 2013 Philipp Meyer book of the same name. He had to learn the Comanche language for the role.
