Lee gained recognition with his role as a North Korean spy in the 2013 film Secretly, Greatly as well as a genius hacker in 2014's The Con Artists,And Money Heist: Korea – Joint Economic Area(2022).

Lee began his career as a child actor, building up his resume by playing roles in a number of large-scale productions, such as The Legend, The Great King, Sejong, The Return of Iljimae, and Queen Seondeok.

[12][13] Despite low ratings during its domestic run, To the Beautiful You led to increased Korean Wave exposure for Lee in countries such as Singapore, Malaysia, Taiwan and China.

[17] His breakthrough continued when he landed a major role in 2013 box office hit Secretly, Greatly, an action-comedy film in which he played a North Korean undercover spy disguised as a high school boy.

[28] In 2017, Lee starred in the drama The Liar and His Lover, the Korean remake of the Japanese manga Kanojo wa Uso o Aishisugiteru alongside Red Velvet's Joy.