Scouted by singer Lee Sun-hee, Lee trained for two years before debuting on June 5, 2004, at the age of 17. “Because You’re My Girl”, his debut song from his first album The Dream of a Moth, became a popular ballad which created a “liking older women” syndrome in South Korea. With this song, he won the “Best Newcomer” award in various music awards ceremonies in 2004, such as the M.net KM Music Festival and Seoul Music Awards. In 2007, he also won the “Best Male Solo Artist” award at the M.net KM Music Festival with his song “White Lie” from his third album Story of Separation.

In 2005, Lee guest-starred in MBC’s sitcom Nonstop 5. He then officially debuted as an actor in KBS’s 2006 weekend drama The Infamous Chil Sisters.

Lee was a permanent member of the first season of 1 Night 2 Days, a segment of KBS’s weekend variety show Happy Sunday, from November 2007 to February 2012. Known by his nickname “Heodang”, his appearance on the show shot him to fame in Korea and made him a household name. In 2008, he received the “Best Popularity” award in the KBS Entertainment Awards due to his popularity with audience in 1 Night 2 Days. He also earned the “Top Excellence Host” award in the 2010 KBS Entertainment Awards and the “Daesang” award with the other cast members of the first season at the 2011 KBS Entertainment Awards.

Lee graduated from Dongguk University obtaining his degree in International Trade and Commerce on February 20, 2009 and received a Special Achievement Award during graduation. He then continued his studies obtaining two master’s degrees, Trade Theory as well as Finance and Cultural Contents at Dongguk Graduate School.

Lee released a digital single titled “Will You Marry Me” in 2009, which became a hit in South Korea. With this single, he received the “Digital Single Bonsang” award in the 24th Golden Disk Awards. The same year, he released his fourth album Shadow, which was a success and received 40,000 pre-order copies before its release.

He was then cast as the male lead in SBS’s weekend drama Brilliant Legacy in 2009, co-starring Han Hyo-joo. The drama maintained its top spot in viewer ratings throughout its run, with its final episode attaining a viewer rating of 47.1%. The huge success boosted Lee’s popularity as an actor and led to increased advertising offers for the actor. With this drama, he received the “Excellence in Acting” award, the “Top 10 Stars” award and the “Best Couple” award with Han Hyo-joo at the 2009 SBS Drama Awards.

Lee took on a hosting role in SBS’s Tuesday night talk show Strong Heart in October 2009, with Kang Ho-dong. It was recognized as the “Best Program” at the 2009 SBS Entertainment Awards and Lee was awarded the “Netizens’ Popularity” award. Lee was again recognized for his role in the talk show and received the “Top Excellence Host” award for two consecutive years at the 2010 and 2011 SBS Entertainment Awards. In March 2012, Lee announced his departure from Strong Heart in order to focus on his singing career. He recorded his last episode on March 15, which was broadcast on April 3.

In 2010, he recorded a duet version of his 2007 song “Smile Boy” with South Korean figure skater Kim Yuna, which became the official 2010 FIFA World Cup commercial song in South Korea. He also received the “Digital Single Bonsang” award for the second consecutive time at the 25th Golden Disk Awards with his song “Love Taught Me To Drink”. This was followed by the release of his fifth album, Tonight in 2011, a departure from Lee’s usual traditional ballads and encompasses more of a rock band-style sound and melodies. The title song “Aren’t We Friends” instantly topped online music charts upon release. His other single, “Era of Love” also earned the No. 1 spot on that week’s Billboard K-Pop Hot 100.

This was followed by SBS’s fantasy drama My Girlfriend Is a Nine-Tailed Fox in 2010, playing a university student who (despite strong discouragement from his grandfather) wants to drop out in order to pursue his dream of becoming an action movie star. His performance earned him his second “Excellence in Acting” award at the 2010 SBS Drama Awards.

On March 6, 2012, Lee made his debut in Japan with the release of his first Japanese album Time for Love (also translated as Alone in Love) and a single of the same title. Both the album and single ranked number one on the Oricon Daily Chart. To celebrate his success, Lee held a fanmeeting at Tokyo Dome City Hall. He held a concert at Nippon Budokan, Tokyo, on June 1, 2012, three months after his debut.

On June 23, 2012, Lee was one of the torchbearers in the 2012 London Summer Olympics Torch Relay chosen by Samsung. On October 30, 2012, the National Election Commission of South Korea appointed Lee as the Honorary Online Ambassador of the nation’s campaign for a fair and clean presidential election.

On June 10, 2015, Lee made his music comeback with his sixth album And… after a hiatus of two years and seven months.

In 2015, Lee made his film debut with romantic comedy Love Forecast which co-stars Moon Chae-won. In July 2015, Lee was cast in the period comedy film The Princess and the Matchmaker with Shim Eun-kyung, directed by The Face Reader director Han Jae-rim. The film was released in 2018.

Lee reunited with 1 Night 2 Days cast members and PD Na Young-seok in the travel-reality show New Journey to the West in 2015.

On January 21, 2016, Lee’s agency, Hook Entertainment, announced that Lee would be enlisting for his mandatory military service on February 1. As a gift to fans, Lee also released a new track on January 21 at noon, titled “I Am Going to the Military”. On March 3, 2016, Lee’s song “Meet Someone Like Me” was released. The single is the last song Lee recorded before joining the army, and was produced by Psy.

On February 1, 2016, Lee began his 21 months of mandatory military service as active duty soldier, entering Nonsan Army Recruit Training Center in Chungcheongnam-do. After training there for five weeks he was dispatched to his assigned role following the completion ceremony. Lee was formally discharged on October 31, 2017 from the military at Jeungpyeong County in North Chungcheong Province where he was a member of the special forces (13th Special Forces Brigade ‘Black Panther’, Republic of Korea Army Special Warfare Command).

In 2017, Lee was cast in the fantasy drama A Korean Odyssey, written by the Hong sisters. This marks his first project after enlistment.

In 2017, Lee was announced to be joining the cast of SBS variety show Master in the House.

In 2018, Lee was announced to be the MC of Mnet’s reality/survival program, Produce 48. The same year, it was announced that Lee will join the second season of Netflix’s variety show Busted!. Lee was nominated and awarded the Grand Award (Daesang) at the 2018 SBS Entertainment Awards for SBS variety show Master in the House. He became the youngest entertainer who win the Grand Award (Daesang).

In 2019, Lee starred in the spy action drama Vagabond.

In 2019, Lee joined the cast of Little Forest, a healing variety program designed as a home kids garden development project for children. The same year, he featured in the Netflix travel show Twogether alongside Taiwanese actor Jasper Liu.

On November 6, 2020, Hook Entertainment announced that Lee will be released a pre-released track titled The Ordinary Man (뻔한 남자) on November 15, ahead of his anticipated 7th album in December 2020