Mieszko was the second son of the Silesian duke Henry II the Pious (1196–1241), by his consort Anne (d. 1265), a daughter of the Přemyslid king Ottokar I of Bohemia.

His father succeeded as Polish high duke in 1238, ruling over Silesia and the Seniorate Province, as well as over large parts of Greater Poland.

He had to ward off several attacks by the Ascanian margraves of Brandenburg and the Magdeburg archbishops on the westernmost Greater Polish lands, stretching around the former Veleti castle of Lubusz on the banks of the Oder river.

This agreement was only effected on a preliminary basis until the younger brothers Henry III the White and Konrad I attained the age of majority.

Shortly afterwards, in 1248/49, Lubusz Land was pledged to the Archbishops of Magdeburg and the Brandenburg margraves by his brother Bolesław II.