When on 9 April 1241 High Duke Henry II was defeated and killed in the Mongol invasion of Poland at the Battle of Legnica, Mieszko and his older brother Bolesław II the Bald were old enough to be considered adults according to the Piast family customs. As contemporary chroniclers called Mieszko with the surname "of Lubusz" ("lubuski"), it seems possible that his older brother Bolesław, while assuming the rule over Silesia, had to cede him the Greater Polish territories of Lubusz as a separate district, probably by the will of their late father.