He was the third and youngest son of Trojden I of Czersk and Maria, daughter of Yuri I, King of Halych-Volhynia.
In 1351, Casimir I increased his patrimony with the district of Sochaczew, which he received after the death of his cousin Bolesław III.
In return of their submission, the Masovian Dukes received the promise that if the King died without sons, the entire inheritance of Bolesław III passed to them.
In fact, they received the main district of Bolesław III, Płock immediately, but in exchange of a payment of 2,000 fines.
Because he never married or had children, his district of Warsaw as an emptied fief was reverted to King Casimir III; however, he agreed to transfer the entire estate to Siemowit III in exchange of a monetary compensation.