He was the son and successor of Peter II of Arborea of the Bas-Serra family and a local woman named Sardinia.

William was the son of Bina de Lacon, widow of Peter I, and Hugh of Capraia, Count of Prato.

In 1270, he imprisoned Nicholas, and in 1274 had him killed and began to govern himself, though he was soon opposed by Berthold's son Anselm, who held Cagliari.

Marianus was a close ally of the Republic of Pisa, the most powerful force on Sardinia in the mid-thirteenth century, and received Pisan citizenship on 17 June 1265.

He was wounded and took refuge in San Leonardo di Siete Fuentes, where, according to some sources, he was poisoned in 1297 by the Pisans who wanted to extend their authority in Cagliari to the Argentiera of Cixerri.