After the death of his father on 10 September 1454, Casimir III and his siblings were placed under the guardianship of their mother Barbara and Paweł Giżycki, Bishop of Płock.
Despite inheriting his own estates, Casimir III decided to enter the Church and, even in his youth, was appointed to the position of Dean and Canon of Płock Cathedral.
Unfortunately, he was opposed by King Casimir IV, who chose his Vice-Chancellor of the Crown and Royal Secretary Andrzej Oporowski as successor, while the Chapter supported their own candidate, Ścibor Bielski.
As a result, he ruled the Bishopric as only a secular administrator until 10 September 1475, when, after the grant of a Papal dispensation for his age, Casimir III was finally installed as Bishop of Płock at Pułtusk Church.
Securing the episcopal office cost him large sums of money, which he then tried to recover from the church treasury, causing understandable opposition from the chapter.