Through her mother, Agnes was a descendant of the Salian dynasty, which ruled the Holy Roman Empire since 1024 until her maternal uncle, Emperor Henry V, died without issue in 1125.

Of her full siblings, one sister, Judith, married Marquess William V of Montferrat, and one brother was Bishop Otto of Freising, a renowned medieval chronicler.

According to Wincenty Kadłubek, Polish chronicler and bishop of Kraków (and this opinion is shared by other sources), Agnes was a very ambitious, energetic woman, and proud of her origins.

The ruler of Poland, Duke Bolesław Wrymouth, in order to hold his ground against the Empire wanted to obtain a powerful ally against King Lothair III of Germany.

In addition, Władysław II would also receive the lands of Łęczyca, then granted by Bolesław III to his widow, Salomea of Berg, for life as her dower and to revert to the Seniorate Province upon her death.

Wincenty Kadłubek stated that the confrontation between Władysław II and his half-brothers was mainly instigated by Agnes, who believed that her husband, as the eldest son, had the right to be the sole ruler of the whole country.

Władysław II escaped to Bohemia, while Agnes and her children remained in Kraków, where for some time they maintained resistance against the junior dukes from the Wawel Castle.

After a short time at the Bohemian court of Duke Vladislaus II, Agnes' half-brother, King Conrad III of Germany, offered his hospitality to the Polish ducal family, who settled at the Kaiserpfalz of Altenburg.

She asked for the intervention of Pope Eugenius III, who decided to raise the question in the 1148 Council of Reims, and sent his legate Guy to Poland to obtain the submission of the junior dukes.