Thieriot was signed to talent manager Don Gibble after taking his improvisation class. He modeled for Gap and appeared in two short films, before being cast in his first feature film role, 2004's adventure film Catch That Kid, alongside Kristen Stewart and Corbin Bleu. He was subsequently cast in 2005's action comedy film The Pacifier, playing one of the family members protected by a Navy Seal played by Vin Diesel. He was nominated for the Young Artist Award for Best Performance in a Feature Film – Supporting Young Actor for his role. Thieriot appeared in the drama The Astronaut Farmer, released on February 23, 2007, as well as in the feature film version of Nancy Drew, released on June 15, 2007.

In 2008, he played a younger version of Hayden Christensen's lead character in Jumper (co-starring with Rachel Bilson and, for the second time, Kristen Stewart. That same year, he appeared in Kit Kittredge: An American Girl, for which he won the Young Artist Award Best Performance in a Feature Film – Young Ensemble Cast with his co-stars. In 2009, Thieriot co-starred in the erotic thriller Chloe, theatrically released by Sony Pictures Classics on March 26, 2010. Chloe enjoyed commercial success and became director Atom Egoyan's biggest ever box office earner. The following year, he appeared in Wes Craven's horror film My Soul to Take, replacing Dennis Hopper's son Henry Lee Hopper in the cast. Thieriot portrayed the film's main character Adam "Bug" Hellerman, who is one of seven characters chosen to die.

In 2011, he starred in the independent comedy The Family Tree, alongside Dermot Mulroney and Britt Robertson, and had the lead role in the drama film Foreverland, starring opposite Juliette Lewis. Thieriot then co-starred with Jennifer Lawrence in the Mark Tonderai-directed psychological horror-thriller House at the End of the Street, which was released to commercial success in 2012. That same year, he starred in the drama film Disconnect. In 2013, Thieriot joined the main cast of the critically acclaimed drama-thriller series Bates Motel, alongside Vera Farmiga and Freddie Highmore. He made his directorial debut on the series with the fifth season episode Hidden.

In 2015, he starred in the History Channel event miniseries Texas Rising as John "Jack" Hays, a captain of the Texas Rangers, and appeared in the remake of the action-thriller film Point Break. Thieriot has been cast as Officer Clay Spenser in the CBS drama series Seal Team, opposite David Boreanaz and Jessica Paré.

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