Ulrich Cillei was the son of Frederick II, count of Cilli, and Elizabeth Frangepan. Of his youth we know nothing certain. About 1432 he married Catherine, daughter of George Brankovich, despot of Serbia.
His influence in the troubled affairs of Hungary and the Empire early overshadowed that of his father, together with whom he was made a prince of the Empire by the emperor Sigismund (1436). Hence feuds with the Habsburgs, wounded in their rights as overlords of Cilli, ending, however, in an alliance with the Habsburg king Albert II., who made Ulrich for a short while his lieutenant in Bohemia. After Albert’s death (1439) Ulrich took up the cause of his widow Elizabeth, and presided at the coronation of her infant son Ladislaus V. Posthumus (1440). A feud with the Hunyadis followed, embittered by John Hunyadi’s attack on George Brankovich of Serbia (1444) and his refusal to recognize Ulrich’s claim to Bosnia on the death of Stephen Tvrtko (1443). In 1446 Hunyadi, now governor of Hungary, harried the Cilli territories in Croatia-Slavonia; but his power was broken at Kossovo (1448), and Count Ulrich was able to lead a successful crusade, nominally in the Habsburg interest, into Hungary (1450). In 1452 he forced the emperor Frederick III. to hand over the boy king Ladislaus V. to his keeping, and became thus practically ruler of Hungary. In 1454 his power was increased by his succession to his father’s vast wealth; and in 1456 he was named by Ladislaus his lieutenant in Hungary. The Hunyadis now conspired to destroy him. On the 8th of November, in spite of warnings, he entered Belgrade with the king; the next day he was attacked by Laszlo Hunyadi and his friends, and done to death. With him died the male line of the counts of Cilli.
Count Ulrich’s ambition was boundless, his passions unbridled; but the hostile judgments passed by Aeneas Sylvius and other contemporaries upon him must be read with caution.