After the fall of Communism, he became a leader of the centre-left Social Democracy of the Republic of Poland, a successor to the former ruling Polish United Workers' Party, and a co-founder of the Democratic Left Alliance.

[7] He became politically active at this time, and joined the ruling Polish United Workers' Party (PZPR) in 1977, remaining a member until it was dissolved in 1990.

A participant in the Round-Table negotiations, he co-chaired the task group for trade-union pluralism with Tadeusz Mazowiecki and Romuald Sosnowski.

[11] As the PZPR was wound up, he became a founding member of the post-communist Social Democratic Party of the Republic of Poland (SdRP) from January to February 1990, and its first chairman until he assumed the presidency in December 1995.

Running for the Sejm from the Warsaw constituency in 1991, he won the largest number of votes (148,533), although did not win an absolute majority.

[15] Political opponents disputed his victory and produced evidence to show that he had lied about his education in registration documents and public presentations.

[citation needed] In 1997, the Polish newspaper Zycie reported that Kwaśniewski had met former KGB officer Vladimir Alganov at the Baltic sea resort Cetniewo in 1994.

He also took an active part in promoting further enlargement of the alliance, speaking out in favor of membership for a further seven states and the open-door policy that leaves open the option of further members.

After a history of sometimes acrimonious relations with Lithuania, Kwaśniewski was a driving force behind the presidential summit in Vilnius in 1997, at which the two countries' presidents signed a treaty of friendship.

[24][25] Thanks to his close relations with Leonid Kuchma, in late 2004 he became a mediator in a political conflict in Ukraine – the Orange Revolution, and according to some commentators, he played the major role in its peaceful solution.

[27] In December 2005, when his presidency was coming to an end, he granted clemency for a post-Communist deputy minister of Justice Zbigniew Sobotka, who had been sentenced for 3.5 years of prison for revealing a state secret (effectively, he warned gangsters about an operation against them).

[31][32] Kwaśniewski refused in 2003 to face a special parliamentary commission,[33][34] which was set up to reveal all circumstances linked with Rywingate.

Kwaśniewski argued, that the constitution did not allow parliamentary commissions to investigate the president, and there were no clear law opinions.

[35] For a second time Kwaśniewski refused as a witness to face the commission investigating the privatization of Orlen petrol concern, in March 2005.

[citation needed] In 2007, the Institute of National Remembrance revealed that Kwaśniewski was registered during communist times as an agent "Alek" of the secret police, the Security Service (Służba Bezpieczeństwa – SB), from 1983 to 1989.

[37] On 7 March 2006, Kwaśniewski was appointed Distinguished Scholar in the Practice of Global Leadership at Georgetown University, where he teaches students in the Edmund A. Walsh School of Foreign Service about contemporary European politics, the trans-Atlantic relationship, and democratization in Central and Eastern Europe.

He also teaches a course on political leadership, convened by Professor Carol Lancaster, with former Spanish Prime Minister José María Aznar.

[40] Since 2011, Kwaśniewski has served on the Leadership Council for Concordia, a nonpartisan, nonprofit based in New York City focused on promoting effective public-private collaboration to create a more prosperous and sustainable future.

[44] In a plea agreement filed in United States Federal court on 14 September 2018, former Donald Trump campaign chair Paul Manafort admitted to organizing a group of former European heads of state to illegally lobby, starting in 2011, on behalf of then-Ukrainian president Viktor Yanukovych.

The plea agreement describes one of the heads of state involved in this secret lobbying as a "former Polish President" who "was also a representative of the European Parliament with oversight responsibility for Ukraine.