He soon occupied Belz, Rawa Mazowiecka and Gostynin, who were formally added to the kingdom; however, the Thirteen Years' War and the fear of an intervention of the Teutonic Knights, united with the disapproval of the Masovian nobility over the too aggressive politics of the Casimir IV, forced him to accept temporarily the rule of Konrad III over Płock, Płońsk and Zawkrze —although the major support of Konrad III was Princess Catherine of Płock, aunt of the late dukes, whose energetic attitude played a major part in the conflict.

The verdict of the court could be easily predicted and on 25 November the outraged duke left the place of the meeting, where was confirmed the incorporation of Rawa Mazowiecka, Gostynin and Belz to the crown, and also stipulated that all the inheritance of Siemowit VI and Władysław II shared the same fate.

For unknown reasons, despite confirmations of a judgment in the following years, Casimir IV didn't exercise his rights over Płock, who remained in the hands of the Piast princes (however, in 1476 Konrad III was forced to accept the annexation by the crown of the town of Sochaczew, who belonged to Anna of Oleśnica, widow of Władysław I, as a part of her Qprawa wdowia).

Not wanting to risk his whole inheritance if he should lose the royal favor, Konrad III decided to support the crown during his war against the Teutonic Order, sending auxiliary troops in 1464 during the siege of Działdowo.

According to earlier agreements, Konrad III inherited Warsaw; however, for unknown reasons, the local townspeople refused to accept him, and called his youngest brother Janusz II to be their ruler.

Being the Jagiellonian dynasty a constant threat to the Masovian princes, Konrad III tried to limit his contacts with countries hostile to the Kingdom of Poland, like the Teutonic Knights, and even Moscow.

John I Albert, in contrast to his father, refused to lose the battle and in August 1495 Polish troops reached Płock; Konrad III, watching the impossibility to resist, decided to give up.

The events of Piotrków Trybunalski limited the internal policy of Konrad III, because from then forward, every nobleman who had a conflict with the duke would be expected to receive the support of the king.

In 1501 the senate of the kingdom almost deprived Konrad III from all his power, when in a controversial ruling certain noble received lands in perpetuity as a payment for some offences inflicted by the duke.