Born around 1048 following his father's succession in Hordaland, Magnus began his military career at a young age, supposedly leading an expedition into the Manx Sea in 1058. Here he became involved in the disputes of the Norse-Gaelic kings and struggles for the title of High King of Ireland, extending Hordaland's influence over the Isles and Dublin. Raids on newly reunified Wales followed, and then on the Wirral peninsula and Lancashire too, taking advantage of disruption in western Anglia. Magnus's intervention in Irish politics do not appear to have made much of a major impact though did set the precedent that this was Hordaland's sphere and owed fealty to Bergen.
In 1066 Harald launched an unsuccessful attempt to conquer Anglia, probably using much of the intelligence Magnus had gathered over the years. Magnus himself was not involved in the invasion and had been made regent of Hordaland while Harald was away. Subsequently when the news that Harald had been killed on the battlefield Magnus was proclaimed king. His younger brother Olaf (who had been in Anglia) was made co-king and the two brothers ruled over Hordaland peacefully.
In 1069 Magnus died, possibly of food poisoning and Olaf became sole king. His only child (by a mistress) Haakon would in time become king of Hordaland, but like his father only reigned briefly.