He was the son of Marquis Boniface III and Maria of Serbia. At the time when he succeeded his father (1494), William was still a minor. At first, his mother acted as regent. After her death (1495), regency passed to his mother's cousin Constantine Komnen-Arianiti (1495-1499).
He continued the pro-French policy of his father and, in 1508, he married princess Anna d'Alençon, daughter of René d'Alençon. During the Italian Wars, in 1513, he protected the French retreat from Milan; however, to avoid retaliation from Maximilian Sforza William was forced to pay him 30,000 scudi.
The Milanese Duke did not respect the treaty, and his troops invaded Montferrat and sacked numerous cities. At the same time, informed that his relative Oddone, Marquis of Incisa, was trying to obtain the title of Marquis of Montferrat, William marched to Incisa and occupied that lordship. Oddone and his son Badone were sentenced to death. However, the annexation was later declared null by Holy Roman Empire diplomacy, but in the subsequent year, William was able to maintain the new lands.
He had 3 children with Anna d'Alençon:
He died in 1518 and was succeeded by his son Boniface IV, initially with his mother Anne d'Alençon as regent.
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