Karácsony worked for the Medián market and public opinion research company as a research manager. He became Director of Research in 2007. In addition he has been a lecturer at the Corvinus University of Budapest since 2004. He became a member of the newly formed Politics Can Be Different party in 2009. During the 2010 parliamentary election he served as campaign manager of the party. He became a Member of Parliament from the Budapestregional list. In May 2010 he was elected deputy leader of the LMPparliamentary fraction. As a result he left the Medián firm. He was the party's candidate at the Budapest District II by-election in November 2011. He came third with 6.45 percent after Zsolt Láng and Katalin Lévai. Both MSZP and LMP agreed that the candidate who received fewer votes would withdraw in favour of the stronger one, however Karácsony also participated in the run-off. In January 2013, the LMP's congress rejected electoral cooperation with other opposition forces, including Together 2014. As a result members of LMP's "Dialogue for Hungary" platform, including Karácsony, announced their decision to leave the opposition party and form a new organisation. Benedek Jávor, leader of the "Dialogue for Hungary" platform, said the eight MPs leaving LMP would keep their parliamentary mandates. The leaving MPs established Dialogue for Hungary as a full-fledged party. In June 2014, Karácsony was elected co-chair of Dialogue for Hungary alongside Tímea Szabó, when his predecessor Jávor became a Member of the European Parliament in the 2014 European Parliament election. Karácsony won the mayoral election in Zugló during the 2014 local elections as a joint candidate of the Hungarian Socialist Party, Democratic Coalition, and the Together 2014–Dialogue for Hungary alliance. According to the new rules, he also became a member of the General Assembly of Budapest. In April 2017, Karácsony was re-elected co-leader of the Dialogue for Hungary and was also appointed as his party's candidate for the position of prime minister in the 2018 parliamentary election. The Hungarian Socialist Party also elected Karácsony as their candidate for the position of prime minister in December 2017. The two parties also decided to jointly contest the 2018 national election. Consequently, the Together has terminated their cooperation agreement with the Dialogue for Hungary. Under the leadership of Karácsony, the MSZP–PM joint list received 11.91% and came only third after Fidesz and Jobbik. In June 2019, in the opposition's first primary election, he was elected as the opposition's (MSZP-P-DK-Momentum-LMP-MLP While Jobbik did not endorse Karácsony outright, the party opted not to run a candidate against him. This left Karácsony as the sole opposition candidate for the position of Lord Mayor of Budapest in the 2019 local elections, against incumbent Lord MayorIstván Tarlós, who was supported by the ruling coalition, Fidesz–KDNP. He then went on to win the election on 13 October 2019 with 50.86% of the votes being cast in his favor with Tarlós receiving 44.10%.
