Lee Dong-gook participated in the AFC U-19 Championship in 1998. He played with Kim Eun-jung as two top and led South Korea to win the title. Also, Lee was the top scorer and MVP of the competition. Lee was a member of the South Korea national team at the 1998 FIFA World Cup. He also took part in the 2000 AFC Asian Cup and finished top scorer with six goals.

Lee started playing football for Pohang Steelers and scored eleven goals with outstanding performance in his first season. He also contributed to team’s title with two winning goals in the 1997–98 Asian Club Championship. He was named the K League Young Player of the Year and made his debut for the South Korea national team. In January 2001, he was sent on loan to Werder Bremen of Bundesliga but he did ot settle in Bremen and only appeared seven matches as a substitute during six months. At the 2002 Korean FA Cup, he led Pohang to the final by scoring in the round of 16, quarter-final and semi-final, but failed to win. In March 2003, he left Pohang to perform his military service, joining the military team Gwangju Sangmu for two years. Early in the 2006 season, his performance was at its height with six goals in seven matches. In April 2006, however, he tore the cruciate ligaments in his knee while playing in a league match that forced him out for six months, and was unable to play in the 2006 FIFA World Cup.

In 2002, Lee was not chosen by Guus Hiddink to represent South Korea in 2002 FIFA World Cup. Fans called him “Lazy Genius”, because they felt that he did not fully use his potential. This is also because Hiddink emphasized strong stamina, great power, and agile speed, which are the categories that Lee struggles in, except power. He later admitted that he spent his days drinking and did not watch a single game that took place during the 2002 World Cup.

Lee married Lee Soo-jin, a runner up of Miss Korea 1997, in December 2005. The couple have five children: twin daughters Lee Jae-si and Lee Jae-ah (born 14 August 2007), twin daughters Lee Seol-ah and Lee Soo-ah (born 18 July 2013) and son Lee Si-an, nicknamed Daebak (born 14 November 2014).

In January 2007, after being granted a work permit, Lee signed for Premier League club Middlesbrough on an 18-month contract. On 24 February 2007, he made his debut, coming on as an 85th minute substitute for Yakubu in the 2–1 win against Reading. On 29 August 2007, he scored his first goal against Northampton Town in the 2007–08 Football League Cup. On 26 January 2008, he scored his second goal against Mansfield Town in the 2007–08 FA Cup. However, he didn’t score during 23 appearances in the Premier League and disappointed English fans. His contract expired at the end of the 2007–08 season and Middlesbrough decided not to extend his contract.

Lee was coach Dick Advocaat’s first-choice selection at forward for Korea ahead of the 2006 FIFA World Cup, but a knee injury suffered in a K League match forced him to miss the tournament. On 1 November 2007, Lee was banned from the national team for twelve months after it was revealed that he, along with team captain Lee Woon-jae, teammates Kim Sang-sik and Woo Sung-yong, went on a late night drinking spree with several female employees during the 2007 AFC Asian Cup, in which Korea received third place. Unlike the other members who were involved in this incident, because Lee played for Middlesbrough in England, the KFA could not ban him from his club team matches.

Lee signed a contract with Seongnam Ilhwa Chunma in 2008. However, he was unsuccessful, and was transferred to Jeonbuk Hyundai Motors in the 2009 transfer window. On 15 March 2009, he scored a brace in his debut for Jeonbuk against Daegu FC. He became the top scorer in the 2009 K League, scoring 21 goals in 29 matches. Jeonbuk won the first league trophy in their history and he was named the Most Valuable Player. At the 2011 K League, he led team’s second league title by recording 16 goals and 15 assists with 29 appearances. He became the Most Valuable Player and the top assist provider. He also led team to the 2011 AFC Champions League Final by scoring nine goals. Jeonbuk lost to Al Sadd after penalty shoot-out, but he was named the Most Valuable Player and top scorer. On 3 March 2012, he became the K League all-time leading scorer with 117 goals when he got two goals in the 2012 season opener against Seongnam Ilhwa Chunma.

In 2013, Lee was involved in a bizarre incident in a K-League match against Seongnam FC when, while attempting to return the ball to Seongnam’s goalkeeper after an injury stoppage, he accidentally struck it past him from 40 yards to level the score at 1-1. Joenbuk goalkeeper Choi Eun-sung then sportingly scored an own goal on purpose to restore Seongnam’s lead.

On 15 March 2016, he scored his 30th AFC Champions League goal against Becamex Binh Duong and became the AFC Champions League all-time top scorer. On 26 November 2016, Jeonbuk won the 2016 AFC Champions League by defeating Al Ain in the final. On 2 June 2019, he scored his 200th career goal at Jeonbuk Hyundai Motors in a match against Sangju Sangmu FC.

Lee was listed onto the national team line up for the 2010 FIFA World Cup. He came on as a substitute in a game against Uruguay and nearly scored a goal but his shot lacked the power to clear the line. He played in qualifiers for the 2014 FIFA World Cup but was not listed on the final team for the tournament. After the World Cup he was recalled back to the team for a friendly match against Venezuela, where he scored a brace in a 3–1 victory, the national team’s first win in nearly six months. In August 2017, Lee was called up for the 2018 FIFA World Cup qualifier games against Iran and Uzbekistan, being one of the oldest players on the squad.

On May 9 2020, Lee scored the only goal in a 1–0 win over Suwon Bluewings, which was notable for being the first international league match, during the global COVID-19 pandemic. On 1 November 2020, he played virtually last game of his 23-year playing career, and got his retirement ceremony immediately after the game. This game was the last round of the 2020 K League 1, and Jeonbuk decided their fourth successive title with 2–0 victory against Daegu FC. On 8 November 2020, he played his last match as a substitute for eight minutes in the final of 2020 Korean FA Cup, getting his last trophy.