Lee Ki-kwang initially debuted as a solo singer under the stage name AJ or Ace Junior before becoming part of the South Korean boy band BEAST in 2009. They released their first extended play Beast Is the B2ST in October of that year. In May 2011, they dropped their first full-length Korean album titled Fiction and Fact. It topped the Gaon Album Chart and sold over 142,000 copies by the end of 2011. In 2017, the group changed their name to Highlight after they moved to a new label. That same year, Lee Ki-kwang resumed his solo musical career and released his first mini album One.

Lee Ki-kwang debuted as an actor in the hit MBC sitcom High Kick Through the Roof in 2009. In 2010, he bagged the Popularity Award at the MBC Entertainment Awards for the variety program Hot Brothers. In 2011, he won Best New Actor at the MBC Drama Awards for his work in the romantic comedy series My Princess. In 2018, he played a rookie production director who can see ghosts in the KBS2 drama Lovely Horribly, which earned him a Best New Actor nomination at the KBS Drama Awards.

Lee Ki-kwang enlisted for his mandatory military service in April 2019. He was discharged in November 2020.