Lee Joon debuted with the South Korean boy group MBLAQ at Rain’s Legend of Rainism concert in October 2009. That same month, they released their debut extended play Just BLAQ, which featured the lead single Oh Yeah. In January 2011, they dropped their first studio album BLAQ Style, which was followed by the release of their Japanese debut single Your Luv in May 2011. In late 2014, Lee Joon left the group to focus on acting. In October 2017, he released his debut solo digital single What I Want to Give to You.

Lee Joon made his acting debut in the 2010 KBS2 teen drama Jungle Fish 2. In 2013, he starred in the film Rough Play, which won him Best New Actor at the 1st Wildflower Film Awards. In 2014, he became part of the main cast of the tvN series Gap-dong, which scored him a Best New Actor nomination at the 7th Korea Drama Awards. In 2015, he appeared in the SBS black comedy series Heard It Through the Grapevine, which was named Best Drama at the 51st Baeksang Arts Awards. He also bagged Excellence Award trophies at the 8th Korea Drama Awards and the 4th APAN Star Awards for his performance in the series. In 2016, he was nominated for an Excellence Award at the MBC Drama Awards for Woman With a Suitcase. In 2017, he joined the main cast of the KBS2 family drama Father Is Strange, which earned him an Excellence Award nomination at the KBS Drama Awards. In 2021, he acted alongside Bae Doo-na and Gong Yoo in the Netflix sci-fi mystery thriller The Silent Sea.

In 2010, Lee Joon revealed in an interview with Newsen that he was suffering from insomnia and bipolar disorder. He was in a relationship with his Father Is Strange co-star Jung So-min. In October 2017, he enlisted as an active duty soldier for his mandatory military service. However, he was transferred to public service in March 2018 due to panic disorder. He was officially discharged in December 2019.