After the death of his father on 10 September 1454, Konrad III and his younger siblings were placed under the guardianship of their mother Barbara and Paweł Giżycki, Bishop of Płock. The regency ended in 1462, when Konrad III attained his majority and became in the legal guardian of his younger brothers. At the beginning of 1462 arrived the news of the deaths of the neighboring Dukes of Płock,
Siemowit VI and
Władysław II without heirs. Konrad III, as the closest male relative and hoping to reunite the whole Masovian lands, immediately assumed the title of Duke of Płock. Unfortunately, King
Casimir IV also put his claims over the lands, under the excuse of being the Polish sovereign they had to reverted to the crown. 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 formal recognition of Konrad III as Duke of Płock, however, didn't end his struggle with the Polish kingdom for this land. In November 1462 the duke was obliged to presented himself at
Piotrków Trybunalski, where he appeared before the royal court led by the Voivode of Kalisz, Stanisław Ostroróg. 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. The majority of his younger brothers forced Konrad III to make a division of their paternal domains on 3 April 1471. As the older brother, he choose first and decided to take Czersk and Liw. This wasn't the final distribution, because in 1474 Konrad III was able to obtain the district of Wyszogród from his brother
Casimir III and ten years later (1484) he also took Zakroczym from his other brother
Bolesław V. On 27 April 1488 Bolesław V, Duke of Warsaw and Nur died without issue. 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. One year later, Konrad III made an agreement with Janusz II, and gave to him the district of Wyszogród in exchange of Warsaw, and in this way the whole inheritance of Bolesław V remained in the hands of Konrad III. 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. However, this didn't avoided another dispute, when after the death of Janusz II on 16 February 1495, his Duchy of Płock was claimed by
John I Albert, son and successor of Casimir IV. Initially Konrad III didn't give up to his rights and instantly locked himself in Płock. 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 successful incorporation of Płock to the Polish kingdom, however, wasn't enough for John I Albert, who in February 1496 ordered Konrad III to presented himself at Piotrków Trybunalski, where he was further humiliated by paying homage to the king for his lands, who after his death would be incorporated to the crown; only the district of Czersk was granted to him as hereditary domain for his descent. 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. Only the death of John I Albert and the ensuing chaos after this caused that this ruling would be exercised. Konrad III died on 28 October 1503 in
Osieck and was buried in
St. John's Archcathedral, Warsaw. ==Marriages and issue==