Kang made his debut in the entertainment industry through popular musical productions The Rocky Horror Show and Grease. Then he played small roles in popular Korean TV Dramas such as Summer Scent (KBS2, 2003), Nonstop 4 (MBC,2003), Youth! Fly To The Sky (KBS2, 2004), More Beautiful Than A Flower (KBS2) and Save the Last Dance for Me (SBS,2004). He started to play lead roles and shot to fame in MBC's 2005 TV Drama Be Strong, Geum-soon!, in which he co-starred with Han Hye-jin. His role in this drama also made him become famous outside Korea, especially in Japan, China, and Taiwan. Meanwhile his first movie debut was in an independent movie in 2005 titled Host and Guest from the Korean director named Shin Dong-il. This movie was acclaimed both in Korean and International Film Festival, including shown in Berlin Film Festival and The 32nd Seattle Film Festival in 2006. He was then cast in the film Rough Cut for which he won several Best New Actor awards for his performance. He played Su-ta, a hot-tempered action star opposite So Ji-sub who played a gangster who wants to be an actor. In 2009 he starred in the action romantic comedy My Girlfriend Is An Agent opposite Kim Ha-neul and won Best New Actor at the Grand Bell Awards. In 2011, he starred with Yoon Eun-hye in the drama Lie To Me, as the wealthy owner of World Hotel, Hyun Ki-Joon. In this drama Gong Ah-Jung (Yoon Eun-hye) tells a small white lie to an old friend saying she is married. When Gong is seen with Hyun later, everyone assumes he is her husband and the rumour starts to spread.