Kim Joon made his entertainment debut with the boyband T-Max in 2007, as a Rapper and a Songwriter for the group. In 2009, Kim became popular after playing the role of F4 member "Song Woo-bin" in the hit Korean drama Boys Over Flowers. Together with his band, their song entitled "Paradise" was featured in the first edition of the Boys Over Flowers Soundtrack. Other songs of T-Max that were featured in the subsequent editions of the Boys Over Flowers Soundtrack were "Say Yes," "Wish You're My Love", "Fight The Bad Feeling," and "Bang Bang Boom".
From April 2009 to December 2010, Kim was a cast member of Invincible Baseball Team, a variety-reality show that aired during Saturday Challenges; he was chosen as MVP for several weeks. In June 2009, he took part in another reality show Mnet Scandal, where a Celebrity dates a non-celebrity for a week. In July 2009, he made his stage debut in the musical Youthful March (젊음의 행진), along with his T-Max co-member Park Yun-hwa. Kim tested positive for H1N1 in November 2009 and had to cancel a fan meeting event in the Philippines.
In March 2010, Kim starred in mobile drama alongside Nao Minamisawa titled Pygmalion's Love, a Korea-Japanese co-production that aired on Japanese mobile TV station BeeTV. In May 2010, T-Max returned to performing with "Don't Be Rude", the first single for their studio album Born to the Max (new members Park Han-bi and Joo Chan-yang replaced Park Yun-hwa after the latter's departure for mandatory military service). The second single was "Words That I Can Say", and Kim appeared alongside actors Oh Ji-ho and Yoon So-yi in the music video.
Kim enlisted for his mandatory military Service on 29 September 2011 for four weeks of basic training, followed by duty as a conscripted policeman. He was discharged on 28 June 2013.