Kim Jung-hoon debuted as a member of the South Korean duo UN in 2000. He then turned to acting after the duo disbanded in 2005. His breakthrough as an actor came in 2006 when he was cast as the second male lead in the hit MBC romantic comedy Goong. That same year, he kicked off a solo singing career in Japan with the mini-album 5 Stella Lights under the name John-Hoon. In 2007, he joined the main cast of the SBS drama Witch Amusement. In 2009, he released his debut solo Korean mini-album called 1st Mini Album. In 2011, he acted alongside Jo Yeo-jung in tvN’s I Need Romance. In 2013, he starred in JTBC’s Her Legend, opposite Choi Jung-won. In 2016, he joined Chinese actor Lin Gengxin and Yoon-ah in the Chinese wuxia series God of War, Zhao Yun. In 2018, he made his Hollywood debut with the sci-fi monster film Pacific Ring: Uprising.

Kim Jung-hoon dropped out of Seoul National University’s School of Dentistry due to his busy schedule in the entertainment industry. He later majored in acting at Chung-Ang University. He enlisted for his mandatory military service in April 2009 and was discharged in February 2011. In 2019, he was sued by his former girlfriend for allegedly violating their loan agreement. The woman claimed that Kim Jung-hoon urged her to get an abortion after he found out about her pregnancy. She also alleged that he failed to fulfill his promise to provide her funding for her housing. The accuser later withdrew her lawsuit against the singer-actor.