Under the stage name Yoo Gun, he began his entertainment career in 1997 as a singer in South Korean boy band Oppa. In 2006, Yoo rose to popularity through his first leading role in the television drama Hello, God! Besides appearing in the TV series Bad Couple, Can’t Stop Now, and Prosecutor Princess, he also made his theater debut in 2010 in the stage play Thief’s Diary.

In 2011, Yoo renounced his U.S. citizenship in order to serve in the South Korean army. He enlisted as an ordinary soldier in March 2011, and was later transferred to the Defense Media Agency in October 2011, where he took over as DJ of the military radio program This is Friends FM Yoo Gun. Yoo also appeared on the KFN (Korean Forces Network) TV program March in 2012.

After his discharge in December 2012, Yoo quickly resumed his career, appearing on the travel program The World is Delicious (오감만족 세상은 맛있다) in an episode about Brazil, followed by a cameo on the melodrama That Winter, the Wind Blows. Yoo then headlined the KBS1 daily drama Sincerity Moves Heaven in 2013.