The son of Erlend II, earl of Orkney, he at first served Magnus III of Norway, who took possession of the islands in 1098, deposing Erlend and his brother, Paul. Paul's son, Haakon, then became regent on behalf of the Norwegian prince, Sigurd, who created Haakon earl in 1105. Magnus had blotted his record with the Norwegians by refusing to fight in Anglesey because of his religious convictions. He was obliged to take refuge in Scotland, but returned to the Orkneys in 1105 and disputed the succession with his cousin Haakon. Having failed to reach an agreement, he sought help from King Eystein II of Norway, who granted him the earldom and he ruled jointly with Haakon until 1116, when war broke out and Haakon took Magnus prisoner. He was kept on the island of Egilsay and executed on April 16, 1117. It was said that Magnus had prayed for the souls of his executioners, and his remains were taken to Kirkwall, where his nephew, Ragnald III, built St Magnus Cathedral, Kirkwall in his memory.