Cimoszewicz was a member of the left-wing Democratic Left Alliance, the Prime Minister of Poland from 1996 to late 1997, the Foreign Minister of Poland in the governments of Leszek Miller (2001–2004) and Marek Belka (2004–2005), the speaker of the Sejm (lower chamber of the Polish parliament) from January to October 2005 and the leftist candidate in the Polish presidential election of 1990 (receiving 9 percent of the vote) and of 2005 (he withdrew before the elections and promised to abandon politics).
Cimoszewicz returned to politics during the 2007 parliamentary election, when he won a Senate seat as an independent candidate. He kept his senator's seat until the end of term in 2015. Since 2015 Cimoszewicz is workstream leader for the Agency for the Modernisation of Ukraine (AMU), where he is responsible for combatting corruption.
On 28 June 2005, Cimoszewicz declared his intent to run for Polish President (see: Election 2005). He instantly became a leader in the polls. He ran previously in 1990 and received 9.21 percent of the vote. In 1990, Lech Wałęsa and Stan Tymiński went on to the second round. Cimoszewicz did not run in the years 1995 and 2000 giving way to his close colleague Aleksander Kwaśniewski who twice became president. His election committee was chaired by the wife of President Kwaśniewski, Jolanta Kwaśniewska.
On 9 July 2005, Cimoszewicz caused a major political uproar by refusing to testify in front of the Orlen commission. He accused seven of its eight members of being politically motivated, partial and bent on undermining his presidential bid. Constitutional experts are split on whether his move was constitutional or if Cimoszewicz broke the law. Fifty-eight percent of Poles disapproved of Cimoszewicz's behaviour before the commission.
According to a poll by Rzeczpospolita, Cimoszewicz was a "hands down" leader on 5 July 2005:
- Cimoszewicz: 28%
- Kaczyński: 19%
- Lepper: 17%
- Religa: 15%
- Tusk: 11%
- Borowski: 5%
He was predicted to win the second round, independent of who was going to reach it from second place. The election was won by Lech Kaczyński.
In 2009, he was one of two candidates to replace Terry Davis as Secretary General of the Council of Europe. However, in September 2009, the Parliamentary Assembly of the Council of Europe elected candidate Thorbjørn Jagland as the new secretary general.