L debuted with the South Korean boy group Infinite in June 2010 with the extended play First Invasion. In July 2011, they released their first Korean studio album Over the Top, which won the Disc Bonsang Award at the 26th Golden Disc Awards. In June 2013, they released their Japanese debut studio album Koi ni Ochiru Toki in two different editions. In 2014, L, Sung-yeol, and Sung-jong were launched as the group’s second sub-unit called Infinite F. In February 2021, L dropped his debut solo single album Memory.

L made his acting debut in the 2011 Japanese drama Jiu: Special Investigation Team. His acting breakthrough came in 2017 when he became part of the main cast of the hit MBC sageuk Ruler–Master of the Mask, which earned him the Best Character Award at the MBC Drama Awards. In 2019, he acted alongside Shin Hye-sun in the KBS2 fantasy romance Angel’s Last Mission: Love, which won him Best New Actor at the KBS Drama Awards. In 2020, he was cast as lead in the KBS2 sageuk Secret Royal Investigator, which scored him a Top Excellence Award nomination at the KBS Drama Awards.

L enlisted as a member of the Republic of Korea Marine Corps for his mandatory military service in February 2021. He has a younger brother named Kim Moon-soo.