He debuted as the lead vocalist of the South Korean boy group BtoB in March 2012.

The same year he made his theatre debut in the Korean musical adaptation of Boys Over Flowers as Goo Jun-pyo.

Prior to his debut as an idol, he won a Daesang at the 16th Suwon Music Festival 2008, and became vice-president of Red Cross Youth Gyeonggi Province in 2009.

[4] Although primarily a vocalist in the group, Lee has also participated in lyric writing on a few tracks by BtoB including, "Last Day", "Killing Me" and "Melody".

which saw each BtoB member releasing solo digital singles during the group's promotional hiatus in the summer of 2017.

[15] His final musical project before military enlistment was "Iron Mask" where he undertook the dual leading role of King Louis XIV as well as his twin brother Philippe [16] In December 2018, he released his first Korean solo EP titled Mark.

He held his first solo concert, SPACE, on January 4, 5 and 6, 2019 at the Yes24 Live Hall in Gwangjin District, Seoul.

[25] On February 21, 2024, Lee released a cover of Can's "Heavenly Fate" as an OST single for the webtoon, A Not So Fairy Tale.

[33][34] In March 2024, Lee halted activities that involved singing due to his "poor throat condition".

[40] On July 24, Lee recorded a special single— a new theme song for the ROK Army alongside Shinee's Onew and Key, Exo's Xiumin, Infinite's Kim Sung-kyu, 2AM's Jo Kwon and Jinwoon, Yoon Ji-sung and actors Kim Minseok, and Lee Jae-kyoon.