In 1970 she joined then-ruling communist Polish United Workers' Party and was a delegate to the 10th PUWP convention in 1987.

After dissolution of the party she joined, as most former members did, newly formed Social Democracy of Poland (SdPl) and later Democratic Left Alliance (SLD).

Sierakowska left SLD with group of other members led by Sejm Marshal Marek Borowski and founded Social Democracy of Poland (SdPl).

SdPl suffered a massive defeat in the 2005 Sejm election and lost all seats, when SLD won fourth place.

She remained one of the SdPl leaders, and ran for Presidency of Lublin, where she had lived for many years, and won first place in the first round with 25.31% of vote.