Stewart appeared as an extra in The WB series Dawson's Creek episode "To Green, With Love". In 2003 he filmed a pilot for a Western for ABC entitled Then Came Jones. In the pilot he played the character of Bill Jenkins.
Stewart later appeared as Sean Cleary the CSI episode "Bad to the Bone", which aired on April 1, 2004. That same year, he also appeared in a commercial for Levi's 501 Original jeans. Stewart currently has a recurring role in Criminal Minds as Detective william LaMontagne, Jr., SSA Jennifer "JJ" Jareau's boyfriend and later, husband.
Stewart has two children with Deanna Brigidi: daughter Ryan Justine (born 2008) and son River Jacob (born 2010).
On February 27, 2012, Stewart went into cardiac arrest after contracting salmonella. His wife performed CPR for several minutes. Stewart now has an implantable cardioverter-defibrillator in his chest.
Stewart got his start doing local theater at the Landmark Theatre in Sutton, West Virginia, and eventually moved to New York City to study at the T. Schreiber studios. He was a company member of the 13th Street Repertory Theatre. He continued his theater in Los Angeles, performing in Light Bulb and Beacon alongside Robert Forster and Brooke Shields.
In 2017, Stewart starred as Solotov on the second season of the USA Network thriller drama Shooter.
On February 26, 2018, it was announced that Stewart will join as a series regular in the second season of Netflix's The Punisher.
