During the 9th century Mazovia was perhaps inhabited by the tribe of Mazovians, and it was incorporated into the Polish state in the second half of 10th century under the Piast ruler Mieszko I. As a result of the fragmentation of Poland after the death of Polish monarch Bolesław III Wrymouth, in 1138 the Duchy of Mazovia was established, and during the 12th and 13th centuries it joined temporarily various adjacent lands and endured invasions of Prussians, Yotvingians, and Ruthenians. To protect its northern section Conrad I of Mazovia called in the Teutonic Knights in 1226 and granted them the Chełmno Land.

The historical region of Mazovia (Mazowsze) in the beginning encompassed only the territories on the right bank of Vistula near Płock and had strong connections with Greater Poland (through Włocławek and Kruszwica). In the period of the rule of the first Polish monarchs of the Piast dynasty, Płock was one of their seats, and on the Cathedral Hill (Wzgórze Tumskie) they raised palatium. In the period 1037–1047 it was the capital of the independent, Mazovian state of Masław. Between 1079 and 1138 this city was de facto the capital of Poland.