Cimoszewicz was a member of the left-wing Democratic Left Alliance the leftist candidate in the Polish presidential election of 1990, receiving 9 percent of the vote.

It was during this time that he, along with Leszek Miller, signed the Accession Treaty that paved way to Polish membership in the European Union.

On 9 July 2005, Cimoszewicz caused a major political uproar by refusing to testify in front of the Orlen commission.

According to a poll by Rzeczpospolita, Cimoszewicz was a "hands down" leader on 5 July 2005: He was predicted to win the second round, independent of who was going to reach it from second place.

However, in September 2009, the Parliamentary Assembly of the Council of Europe elected candidate Thorbjørn Jagland as the new secretary general.

If we do not stop that aggressive [Russian] policy at an early stage then we can face a much more difficult situation requiring more efforts taking higher risks...