One of his first TV appearances was in the third season of MTV's Undressed and the MTV film Everybody's Doing It. In 2002, Harrison appeared as Lonny in the movie Orange County. His first major role was dorky next-door neighbor Brad O'Keefe on Grounded for Life. In 2004, Harrison starred as Green Graves in the coming of age drama film Lightning Bug.
Big Japan's first release, Music for Dummies, was digitally released through Nightshift Records on August 23, 2005. This CD originally titled Music for Dummies was changed to Untitled thus the limited number of CDs printed have become Collectors items.
Harrison appeared in five episodes of That '70s Show as Charlie Richardson, a character intended to be a replacement for departing cast member Topher Grace but Harrison decided to work on another series. He appeared in The O.C. as Danny, Seth's rival. He had a dramatic guest role on Law & Order: Special Victims Unit as Sam Cavanaugh, a young victim of sexual abuse. In film, he appeared alongside Jack Black and Colin Hanks in Orange County. In addition, Harrison also starred in the movie Deal, which was released in the US on April 26, 2008.
During 2006-07 he starred in The Loop, as a young professional trying to balance the needs of his social life with the pressures of working at the corporate headquarters of a major U.S. airline. It was canceled following the second season in 2007. Later in 2007, he became the star of The CW series, Reaper as Sam Oliver. Reaper was canceled on May 19, 2009, after two seasons.
He starred as Scottie Smith in the road trip comedy film Mardi Gras: Spring Break. Also in 2011, he was signed by Fox to fill a main cast role in a mid-season replacement comedy called Breaking In.
Harrison began dating Lauren Zelman in 2005. They married on March 25, 2012.
In 2015, Harrison signed on to play Gordon Cooper on ABC's The Astronaut Wives Club and played the role of Barry Burwood in movie See You in Valhalla.
In 2016, Harrison had a recurring role as Kenny in the Netflix sitcom series The Ranch. In 2017, Harrison guest starred as Brad in an episode of Mom and in 2018, he guest starred as Sam Oliver in an episode of Kevin (Probably) Saves the World.
