Poston began his career in the cast of the Broadway Musical Chitty Chitty Bang Bang, acting in over 300 performances. In 2003, he had a small part playing a young main characters in the film Girls Will Be Girls. Poston played in the 2007 film Walk Hard: The Dewey Cox Story and he played Tommy Huff in the 2008 film Step Brothers with Will Ferrell and John C. Reilly. He plays Trevor in Disneys Kickin It.
Poston had a starring role as Joshua Dowd in James Vasquez's 2008 award-winning feature film Ready? OK!, with Carrie Preston and Michael Emerson.
On 12 October 2010, Poston released his debut single "Knee Deep In Mud".
In November 2011, Poston guest-starred in the season finale of Workaholics, playing Damien Carmichael.
