In the early 1990s, he founded and operated a guiding school in
Mapo District, Seoul, and was recruited by the
National Congress for New Politics, where he began his political activities. In 2007, along with
Lee Jae Myung, he led the establishment of People in Touch with Jung Dong-young, a support group for Jung, and served four terms (17th, 19th, 21st, and 22nd) as a member of the
National Assembly, representing Mapo B, Seoul. During the
17th National Assembly, he led an occupation of the
National Assembly Building to demand the repeal of the
National Security Act. On 10 March 2016, he was not nominated by the
Democratic Party of Korea for the 20th general election due to various controversies and was cut along with several other active members of the party. He said that he respected the party's decision and would not defect, but would help the party in various ways to win the general election. He recommended Son Hye-won, the party's public relations chairperson, for his district, and she was selected as the Democratic Party of Korea's candidate for the 20th general election in
Mapo District. In the
2020 legislative election, he was nominated as the Democratic Party of Korea's candidate for Mapo District and won a third term, defeating Kim Sung-dong of the
People Power Party with 53.75% of the vote. In August 2022, he was elected as a senior member of the Democratic Party of Korea. In the 2024 South Korean legislative election, he ran for the same district and won a fourth term with 52.44% of the vote, defeating Ham Un-kyung of the People Power Party. In the 22nd National Assembly, he was elected chairman of the Judiciary Committee for the first half of the term. He served as impeachment prosecutor in the
Constitutional Court impeachment trial of President
Yoon Suk Yeol. On 2 August 2025, Jung was elected as chair of the Democratic Party of Korea during its national convention in
Goyang,
Gyeonggi Province. He defeated his rival
Park Chan-dae with 61.74% of the vote. Within the party, while he is a part of pro-Lee Jae Myung faction, he takes a hardline stance against the
People Power Party, emphasizing that the opposition party should apologize and reflect on "attempt to destroy the constitution during the
2024 South Korean martial law crisis and kill the people" before cooperation.{{cite web|url=https://koreajoongangdaily.joins.com/news/2025-08-04/opinion/editorials/New-DP-leader-signals-hardline-stance-ignoring-bipartisan-dialogue/2367532|title=New DP leader signals hard-line stance, ignoring bipartisan dialogue ==Controversy==