Kang became a member of CNBLUE in 2009. The group debuted in Japan with the mini album Now or Never on August 19, 2009 with an indie label AI Entertainment. They officially debuted in Korea in 2010 with the mini album Bluetory, and the title track "I'm a Loner" became an instant hit in Korea.
In 2011, he was cast in MBC's youth drama Heartstrings, which stars fellow CNBLUE member Jung Yong-hwa. He sung an OST for the drama, titled "Star".
In 2012, he starred in KBS's family drama My Husband Got a Family. The series topped the weekly ratings chart for 25 consecutive weeks and reached a ratings peak of 45.8 percent (TNmS) and 52.3 percent (AGB Nielsen), making it the highest rated Korean drama in 2012. Kang received his first acting award, the "Rising Star Award" at the APAN Star Awards for his role.
Kang continued to gain more popularity in The Heirs (2013), a teen drama penned by Kim Eun-sook. Along with his co-star Krystal Jung, they were voted as the Best Onscreen Couple at the 2013 DramaFever Awards.
In 2015, Kang became a fixed cast of KBS's reality-variety show Brave Family, and became a host of JTBC's show Mum is Watching.
After a three-year hiatus, Kang made his television comeback in SBS's Entertainer in 2016, for which he won an Excellence Award at the SBS Drama Awards. The same year, he became a host of KBS's live music show Music Bank with Laboum's Solbin from July 2016 to November 2016.