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, King Sejong the Great, The Return of Iljimae, and Queen Seondeok. He rose to fame for his role in Master of Study, a Korean screen adaptation of Japanese manga Dragon Zakura. The same year, Lee took to the stage for the 2010 musical production of Footloose, where he played the leading role of Ren. In 2011, he and K-pop singer Yoon Doo-joon (from B2ST) began hosting the music program Music on Top for cable channel jTBC. He was then featured in IU's music video for the song "You and I". Lee further expanded his acting scope in historical drama Gyebaek, revenge thriller Man from the Equator, and made a guest appearance in medical drama Brain.
In 2012, Lee co-starred in To the Beautiful You, the Korean television adaptation of Japanese manga Hana-Kimi, with his role based on the character Shuichi Nakatsu. 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. He and co-star Hwang Kwang-hee were later announced as the new hosts of live music show Inkigayo. 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. Lee also sang "Ode to Youth" for the film's soundtrack. He became the first male celebrity to be appointed ambassador of the Puchon International Film Festival.
In 2014, he and real-life best friend Park Ji-bin (both were child actors and belong to the same agency) appeared in the travel/reality show Real Mate in Saipan. Lee next played a hacker in the heist movie The Con Artists. He then starred in Northern Limit Line, a 2015 naval thriller about the Second Battle of Yeonpyeong. The film sold 6,024,894 tickets (grossing US$38,945,231), making it the most-watched Korean film in 2015.
In 2016, he starred in the fantasy youth drama Moorim School. Initially slated for 20 episodes, the drama was reduced to 16 episodes instead due to low ratings.
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.
In 2018, Lee released a digital single titled "Twenty-Six", prior to joining the army.
On February 19, 2018, Lee began his mandatory military service as an active duty soldier. He completed his military service and was discharged on October 19, 2019.
|A Dirty Carnival||young Min-ho|
|2007||Hwang Jin Yi||young Nomi|
|2010||The Loneliness of Butcher Boy||Tae-shik|
|2013||Secretly, Greatly||Rhee Hae-jin|
|2004||Oolla Boolla Blue-jjang|
|2006||Hwarang Fighter Maru||Seo Da-ham|
|2007||The Legend||young Cheoro|
|2008||The Great King, Sejong||Prince Choong-nyeong|
|Four Colours of Love||Lee Jung-wook|
|2009||The Return of Iljimae||Cha Dol-yi|
|Queen Seondeok||young Kim Yushin|
|What's for Dinner?||Tommy|
|2010||Master of Study||Hong Chan-doo|
MUSIC VIDEO APPEARANCES
|2011||"You and I"||IU|
|2012||"First Love's Melody"||Acoustic Collabo|
|2013||"I Love You”||Akdong Musician|
|2019||"Above the Time"||IU|
|December 8, 2011 – March 14, 2012||Music on Top||jTBC|
|December 16, 2012 – January 26, 2014||Inkigayo||SBS|
|2012||Invincible Youth Season 2||KBS2||Ep. 26|
|Strong Heart||SBS||Ep. 142, 143|
|2013||Running Man||Ep. 147, 225|
|Hwasin – Controller of the Heart||Ep. 22|
|2014||Real Mate in Saipan||QTV||with Park Ji-bin|
|Running Man||SBS||Ep. 225|
|2015||Happy Together 3||KBS2||Ep. 401|
|2016||Hello Counselor||Ep. 257|
AWARD AND NOMINATIONS
|2008||KBS Drama Awards||Best Young Actor||King Sejong the Great||Won|
|2012||Best New Actor||Man from the Equator||Nominated|
|SBS Drama Awards||New Star Award||To the Beautiful You||Won|
|2013||22nd Buil Film Awards||Best New Actor||Secretly, Greatly||Nominated|
|34th Blue Dragon Film Awards||Nominated|
|SBS Entertainment Awards||Best Newcomer Award||Inkigayo||Nominated|
|2014||9th Asia Model Festival Awards||New Star Award||Secretly, Greatly||Won|
|16th Seoul International Youth Film Festival||Best Young Actor||Nominated|
|2015||52nd Grand Bell Awards||Best New Actor||Northern Limit Line||Nominated|
|30th Korea Best Dresser Swan Awards||Rising Star Award||Won|
|2016||49th WorldFest-Houston International Film Festival||Best Supporting Actor Award||Won|