He was the son of High Duke Leszek I ("the White"), who was assassinated in 1227. In the following internal struggle for supremacy among the Piast dynasty, Duke Henry I ("the Bearded") of Silesia prevailed, who ruled, unchallenged, the Seniorate Province at Kraków from 1232 onwards, and vested minor Bolesław with the Sandomierz duchy once held by his father Leszek. Several years after the death of High Duke Henry II ("the Pious") at the 1241 Battle of Legnica, Bolesław with the support of the Lesser Polish nobility prevailed as prince at Kraków against his uncle Konrad I of Masovia, and thus became the predominant ruler in fragmented Poland.
In 1239 he married Kinga (also known as Cunegunda), daughter of King Béla IV of Hungary. According to medieval chronicles, the marriage was never consummated. Kinga, extremely pious, was averse to fulfilling her marital duties. At first Bolesław tried to change her mind, but she demurred and he reluctantly accepted the situation. His religious convictions forbade him to take a mistress, hence the reference to him as "the Chaste".
After his refusal to participate in the Battle of Chmielnik in 1241 during the Mongol invasion of Poland, the city of Kraków was destroyed but then rebuilt after the Mongols withddrew, and mainly settled by German immigrants in the course of the Ostsiedlung. The new city was vested with Magdeburg rights and rebuilt on a regular grid pattern (the "location" of 1257). During his rule there was a second Tatar raid of the Golden Horde against Poland in 1259, when the lands were devastated and Sandomierz, Kraków and other cities were plundered by the invading forces led by Nogai Khan. Bolesław also had to deal with the claims of his Upper Silesian cousin Duke Władysław ("of Opole"), culminating in an armed conflict in 1273. Władysław was defeated, but the prestige of the title of High Duke was severely eroded. As Bolesław had no heirs, he bequested the Seniorate Province to his nephew Leszek II ("the Black"), the eldest son of his cousin Duke Casimir I of Kuyavia.