William was the only child of Ulrich I, Count of Celje and Adelaide of Ortenburg. The date of his birth is unknown, but was probably in the early 1360s. After his father's death in 1368, William was raised by his uncle Hermann I of Celje. Hermann ruled in his nephew's name, and upon William's coming of age, they ruled jointly. After Hermann's death in 1385, William continued to rule with his cousin, Hermann's namesake son. This was in accordance with the family tradition which avoided splitting the estates by having male members of the family rule together. William also continued the family tradition of allegiance to the Habsburgs who had become the Celjes' liege lords in the early 1300s. It was, however, during his rule that the Counts of Celje strengthened their positions by developing family and political connections stretching far beyond their native region, thus rapidly emancipating themselves from the Habsburg tutelage. Together with his uncle Hermann I and his cousin Hermann II, William took part in the crusade against Samogitians in 1377, as part of the entourage of Duke Albert III of Austria. Like his father and grandfather before him, he served as Landeshauptmann of the Habsburg Duchy of Carniola between 1389 and 1390. In 1387, he was enfiefed with the castle and estate of Dravograd by the Habsburg duke Albert III, further expanding the Celje's possessions along the Drava River in southern Carinthia. William fell ill during an expedition against the Ottomans. He died in Vienna, and was buried in the Minorite Church in Celje.
William's uncle and caretaker Hermann was the brother-in-law of the Hungarian king Louis of Anjou. In 1370, Louis succeeded his uncle Casimir the Great as King of Poland. Casimir had died without sons, he did however have two daughters who survived into adulthood. One of them, Anna, was still alive at the time of her father's death. At the time of Louis's ascension to the Polish throne, Anna was only four, and thus in no position to threaten Louis's claim. In order to neutralize any possible future threat from her part, Louis, who was now her caretaker, decided to betroth her to a loyal, faraway and relatively unimportant ally. The House of Celje could only benefit from marrying a princess of royal blood, but they were in no position to challenge Louis's kingship. In 1380, at the age of fourteen, Anna was married to William. In 1383, Louis, too, died without a male heir. In Hungary, his older daughter Mary would be crowned as "king", while in Poland, his younger daughter Jadwiga became the queen regnant. In 1385, Jadwiga would marry the Lithuanian dukeJogaila who was crowned as Vladislav II of Poland. In 1399, queen Jadwiga died after giving birth to her only child who also died shortly after birth. Vladislav was thus left as the sole ruler of a foreign country, whose legitimacy was contested as he had attained kingship through marriage alone. In order to strengthen his position, Vladislav married William's daughter Anna, the only surviving granddaughter of the last Piast king Casimir the Great. William was thus both the son-in-law and the father-in-law of a Polish king; however, Casimir III died before his marriage to Anna of Poland, and her daughter's marriage to Vladislav II took place after his death.
