Casimir was the third son of King Casimir IV Jagiellon of Poland and Elizabeth of Habsburg — a brilliant prince of the rising Jagiellon dynasty that ruled Poland, Lithuania, Bohemia, and Hungary at its peak. Educated by the great Polish historian Jan Długosz and the humanist Filippo Buonaccorsi (Callimachus), he showed from boyhood a markedly austere and devout disposition unusual in his family.
At thirteen he was offered the throne of Hungary by an anti-Habsburg faction; he refused to take it by force after his initial expedition failed, returning home for the rest of his life convinced that the throne should be God-given. He lived in his father's court as effectively co-regent of Poland from 1481, sleeping on the floor in penance, fasting severely, refusing the customary luxuries of his rank, and (according to all his biographers) maintaining lifelong chastity despite repeated political-marital negotiations on his behalf.
He contracted tuberculosis and died at Grodno on March 4, 1484, at twenty-five. The cult began immediately at his tomb in Vilnius Cathedral, where his body was found incorrupt at examination in 1604. He was canonized in 1521 by Leo X — the first major canonization of a Slavic royal saint since Stanislaus of Szczepanów in 1253. He is the principal patron saint of Lithuania (where his cult is most intense) and a co-patron of Poland.