Marianus' sister Iurgia (Giorgia) was in 1210 married to Emanuele Doria, heir of the Genovese fortress in Logudorese coast.

Pope Honorius III, enemy of the Pisans, immediately sent his chaplain Bartolomeo to annul the marriage, but he failed and the pact between Pisa and Logudoro stood.

They desired to maintain a condominium in Arborea, but internal fighting allowed Peter to solidify his authority with little opposition.

From 1230 to 1232, Marianus exercised the regency of Cagliari on behalf of the young William II and in right of his wife Agnes.

Upon Barison's death (1236) without heirs, the Logudorese magnates, as specified by Marianus' will, had to elect one of his daughters Adelasia or Benedetta (who is thought to have married the Catalan Count of Empuries) to inherit.