Joo Ji-hoon made his debut as a model in 2003. As an actor, Joo Ji-hoon starred in various minor roles in television dramas before finally landing his first lead role in the 2006 romantic comedy series Goong (a.k.a. Princess Hours), which was based on the manhwa Goong. The drama received immense popularity in both South Korea and across Asia, solidifying Joo Ji-hoon’s status as a Hallyu star. He also collected the Best New Actor award at the 2006 MBC Drama Awards for his work in the drama. In 2008, Joo Ji-hoon made his movie debut with Antique, which was based on the manga Antique Bakery.
On April 27, 2009, Joo Ji-hoon was arrested by the Seoul Metropolitan Police on suspicion of supplying ecstasy. He pleaded guilty to using ecstasy and ketamine. He was sentenced to six months in jail, suspended for one year, ordered to do 120 hours of community service, and fined with ₩360,000.
In 2017, Joo Ji-hoon saw resurgence in his acting career. From 2017 to 2018, he starred in the two-part fantasy action film Along With the Gods. He also starred in the critically acclaimed thriller movies The Spy Gone North and Dark Figure of Crime in 2018. In 2019, Joo Ji-hoon returned to the small screen through the Netflix zombie sageuk drama Kingdom.