After graduating from college in 1986, Lee passed the bar exam and entered the Judicial Research and Training Institute to become a lawyer.

In 1995, Lee initiated a civic movement by founding the 'Seongnam Citizens' Association', which aimed to address community issues and promote social activism.

Around 2002, Lee led a movement to establish a new municipal hospital in Seongnam after two general hospitals closed down, highlighting his commitment to public welfare.

In the local elections of 2006, Lee contested as a candidate but faced defeat, attributed to negative public sentiment towards the Uri Party and the Roh administration.

In the 2007 presidential election, Lee Jae-Myung held the position of senior deputy chief in the campaign office of Presidential Candidate Chung Dong-young, showcasing his involvement in national politics.

During the 2008 general election, Lee applied for a nomination in the Seongnam Jungwon A constituency but was defeated, indicating the challenges he encountered in securing a political position.

Lee entered politics in 2010 after working as a human rights lawyer. His political career has been marked by policies aimed at addressing inequality and youth unemployment.

As an animal rights advocate, Lee made the decision to ban dog meat and shut down dog slaughtering facilities in Moran Market. This move was significant due to the moral, ethical, and environmental debates surrounding the issue.

In September last year, Lee avoided arrest when a court rejected a prosecution request to detain him before the trial for multiple corruption charges.

In 2018, Lee Jae-myung assumed office as the 35th Governor of Gyeonggi Province in South Korea after a successful run in the election.

On 1 March 2018, Lee Jae-myung suggested that Japan, which he called 'an aggressor country', should have been divided into two countries after the Pacific War, criticizing the division of Korea instead.

Lee received praise for his handling of the COVID-19 pandemic as Governor of Gyeonggi Province. He took action against the founder of the Shincheonji Church of Jesus, who refused testing and cooperation in contact tracing.

In July 2020, the Supreme Court ruled Lee Jae-Myung not guilty of breaching campaign law during a television debate for Gyeonggi provincial governor, despite allegations of distorting facts.

Lee Jae-Myung announced a comprehensive foreign policy plan emphasizing the improvement of the quality of the people and enhancing national interest.

On October 10, 2021, Lee expressed his support for New Deal liberalism and Franklin D. Roosevelt's policies, aiming to change the economic growth trajectory with a state-led economic revival policy.

Lee Jae-Myung expressed admiration for former dictator Park Chung Hee for creating the Gyeongbu Expressway and announced plans for an 'energy highway' during his administration.

Lee Jae-Myung distributed an article stating that if he differentiates himself from the Moon Jae-in government's feminist-first policy, he can gain support from young men.

Lee Jae-Myung shared a post on his Facebook page criticizing the 'feminism of madness' of the Moon Jae-In government.

Lee Jae-Myung faced criticism from South Korea's liberal camp for evaluating Chun Doo-hwan's economic performance favorably, leading to accusations of aligning with conservative views.

In 2022, Lee Jae-myung was chosen as the Democratic Party's candidate for the South Korean presidential election, although he narrowly lost in the general election to Yoon Suk Yeol.

In January 2024, Lee Jae-myung survived a stabbing incident in the neck during an assassination attempt in Busan while campaigning for the 2024 legislative elections.

On 2 January 2024, Lee Jae-Myung was stabbed in the left side of his neck by an assailant wearing an 'I'm Lee Jae-myung' paper crown. The assailant lunged at Lee during a question and answer session at a construction site in Busan. Although the neck wound was not life-threatening, Lee underwent surgery due to damage to his jugular vein.

Lee Jae-Myung, the leader of the main opposition party, is anticipated to run for the presidency in 2027. He has been a vocal critic of President Yoon.