[3][4] The lands of the Masovians east of the Vistula river had been conquered by the Piast duke Mieszko I of Poland (960–992) and formed a constituent part of his Civitas Schinesghe.

In 1233 Konrad gave Kujawy to his second son Casimir I, while Masovia passed to the first-born Bolesław I upon his death in 1247, succeeded by the youngest brother Siemowit I the next year.

When the kingdom was finally restored in 1295 by the coronation of Duke Przemysł II of Greater Poland, the Duchy of Masovia remained independent.

Upon the death of Duke Boleslaus II in 1313, Masovia was divided among his sons: As neither Siemowit II nor Bolesław III of Płock left any heirs, Trojden's son Duke Siemowit III (1341–1381) was able to re-unite most of the Masovian lands under his rule; in 1351 he and his brother Casimir became vassals of the Polish kings, while the Bishopric of Płock had always been part of the Polish Archdiocese of Gniezno.

After the establishment of the Rawa and Płock Voivodeships, in 1495 the last surviving son of Boleslaus IV, Duke Konrad III Rudy, once again united the remaining Masovian lands under his rule.