In 1171 he was declared adult and until then the government was controlled first by the chancellor Stephen du Perche (1166–1168), cousin of Margaret, and then by Walter Ophamil, archbishop of Palermo, and Matthew of Ajello, the vice-chancellor.

In 1174 and 1175 William made treaties with Genoa and Venice and his marriage in February 1177 with Joan, daughter of King Henry II of England and Duchess Eleanor of Aquitaine, marks his high position in European politics.

Afterwards while the army (allegedly 80,000 men including 5,000 knights) marched upon Thessalonica, the fleet (200 ships) sailed towards the same target capturing on their way the Ionian islands of Corfu, Cephalonia, Ithaca and Zakynthos.

[9] The troops then marched upon the capital, but the army of the emperor Isaac Angelus, under the general Alexios Branas, defeated the invaders on the banks of the Strymon (7 November 1185).

His admiral Margarito, a naval genius equal to George of Antioch, with 60 vessels kept the eastern Mediterranean open for the Franks, and forced the strong Saladin to retire from before Tripoli in the spring of 1188.

[1] In November 1189 William died at Palermo, leaving no children,[1] although Robert of Torigni records a short-lived son in 1181: Bohemond, who was named Duke of Apulia.

[citation needed] After his death Norman officials led by Matthew of Ajello supported his cousin Tancred to succeed him, instead of Constance, in order to avoid German rule.