He was the fifth son of Bolesław IV of Warsaw and Barbara Olelkovna of Slutsk-Kapy, a Lithuanian princess, (granddaughter of Vladimir Olgerdovich). The premature death of three of his older brothers during 1452-1454 left him as the second surviving son of his family.
In the division of the paternal domains, he was given Płock, Płońsk, Wizna, Wyszogród and Zawkrze. 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. After the death of Ścibor z Gościeńczyc, Bishop of Płock, on 4 May 1471, Casimir III announced his candidacy for the post.
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.
However, when the chapter realized that they could not impose their own candidate, they decided to support Casimir III. The case was finally settled in Rome, where, despite the vigorous intrigues of the Polish king, the Pope finally decided in favour of Casimir III, thanks in part to the major support who received from Frederick III, Holy Roman Emperor (who through his mother Cymburgis was related with the Masovian Piasts). The election of Casimir III took place despite his not being the minimum age for a canonical post - 30 years.
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. Casimir III was not able to make a great impact as a bishop.
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.
The case went to the Pope, who helped the diocese taking his finances under his direct control. Soon after, Casimir III died of the plague on 9 June 1480 at Pułtusk, where he was buried.
