He does not appear to have been taken as king after his father's death together with his brothers Sigurd and Inge.
He first appears in the sagas in 1142, when Harald's oldest son, Eystein, came from Scotland and became king, together with Magnus.
In a poem by the skald Einar Skuleson, all the four are counted as kings at the same time.
In contrast to his three brothers who are praised for deeds as warriors, Magnus is said to "make peace between the men".
According to the sagas, Magnus had poor feet, and he became sick and died at a young age.