He was the second son of King Tancred of Sicily and Sibylla of Accera. At the age of four, shortly after the death of first his older brother, and then a few weeks later of his father (February 20, 1194), William was crowned king by Pope Celestine III in Palermo. He would be the last of Sicily's Norman kings. His mother Sibylla, acted as his regent.
But the Emperor Henry VI claimed the throne of Sicily in right of his wife Constance, who was William's great-aunt. Even before Tancred's death he had been laying plans to invade, and his resources had been further augmented by the ransom paid by Richard I of England.
Sibylla was unable to organize much effective resistance. By the end of October of 1194 Henry had conquered all the mainland parts of the kingdom and crossed over into Sicily. On November 20 Palermo fell.
Henry offered Sibylla generous terms: William was to retain the county of Lecce, which had been his father's before he had become king, and was also to receive the principality of Taranto. With that agreement reached, William, his mother and his sisters watched while Henry was crowned king of Sicily on December 25.
Four days later, a conspiracy against the new king was uncovered, and many of the leading Sicilian political figures were arrested and sent to prison in Germany, including William and his family.
While his mother and sisters were eventually released and lived obscurely in France, nothing is known for certain of William's subsequent fate. Some say he was blinded, castrated, or both. Some say he died in captivity a few years later, others that he was released and became a monk. Another theory is that he later returned to Sicily under the alias Tancredi Palamara. Henry's son, Emperor Frederick II (who was also king of Sicily) discovered Tancredi Palamara in Messina and had him executed in 1232.