A foreigner and a Catholic by birth, he was the son of Trojden I, Duke of Masovia and a member of the Polish Piast dynasty.

The name of Bolesław's wife is disputed; Teodor Narbutt claimed her pagan name was Eufemia and her Christian name was Maria.

[2] In a treaty of 1338 Yuri ІІ offered Casimir III of Poland succession to the throne of Galicia-Volhynia.

[3] He was poisoned in 1340 by Orthodox Ruthenian boyars and died without an heir, before his father who continued rule the Duchy of Mazovia.

Yuri's murder sparked the Galicia–Volhynia Wars fought in the years 1340–1392 over the succession in the Kingdom of Ruthenia.