Lee Jong-hyun debuted with the pop rock band CNBLUE in 2009 with the Japanese extended play Now or Never. In January 2010, they made their Korean debut with the mini-album Bluetory, which produced the hit single I’m a Loner. In March 2010, they dropped their debut Japanese studio album called Thank U. A year later, they released their first Korean full-length album titled First Step, which featured the singles Intuition and Love Girl.

Lee Jong-hyun made his acting debut in the 2010 omnibus movie Acoustic. In 2012, he was cast in his first small screen role in the SBS series A Gentleman’s Dignity, which won him the New Star Award at the SBS Drama Awards. The song My Love that he recorded as part of the drama’s official soundtrack was also selected as Best OST at the 22nd Seoul Music Awards. In 2015, he joined the main cast of the KBS2 series Orange Marmalade. In 2017, he starred in Netflix’s My Only Love Song and in KBS2’s Lingerie Girls’ Generation. In 2018, he acted alongside Kim So-eun in the OCN fantasy rom-com That Man Oh-soo.

In 2016, Lee Jong-hyun and his bandmate Jung Yong-hwa were caught in an insider trading controversy. The latter was cleared of suspicion while Lee Jong-hyun was fined with ₩20 million. In 2019, he was involved in the Jung Joon-young KakaoTalk chatrooms issue, which was part of the Burning Sun scandal. He admitted to watching sex videos shared by Jung Joon-young in a chatroom and making degrading comments about women. A few months later, he was once again swept in a controversy after a female YouTuber revealed that she received sexually suggestive direct messages from him over Instagram. Amid renewed backlash, he departed CNBLUE.