He was a legislator and a warrior whose reign saw the commencement of massive codification of the laws of his realm and incessant warfare with the Crown of Aragon.

In 1336 in Barcelona, he married Timbora, daughter of Dalmatius IV of Rocabertí and Beatrice of Serrallonga, Baroness of Cabrenys.

Though an alliance had been in effect with Aragon for more than fifty years at the time of his accession, Marianus realised that the political aim of Peter IV was nothing less than the annexation of Sardinia and, following the conquest of Alghero (1353), he parted ways with the Aragonese.

His armies proved successful in the field and he succeeded in expelling the Aragonese from every redoubt on the island save the stronghold of Castel di Castro.

Peter IV, however, sent a fleet commanded by Pere de Luna to lead an Aragonese army deep into Arborea, bypassing other fortifications to assault Oristano.

The Aragonese troops were trapped between the Arborean armies commanded by Marianus and his son, the future Hugh III, and defeated.

The chief legislative work of his lifetime was the Codice Rurale, which his daughter Eleanor later incorporated into her massive Carta de Logu.