Magnus the Blind was the son of King Sigurd Jorsalfar of Norway and Borghild Olavsdotter.

When Sigurd Jorsalfar died in 1130, Magnus became king of Norway together with his uncle Harald Gille. After four years of uneasy peace, Magnus openly began to prepare for war. 9th August 1134, he defeated King Harald in a decisive battle at Färlev in Bohuslän, and Harald fled to Denmark. Against the advice of his councillors, Magnus disbanded his army and travelled to Bergen to spend the winter there. Harald returned from Denmark with a new army. Meeting little opposition Harald reached Bergen before Christmas. Magnus had few men, and the city fell easily to Harald's army the 7th January 1135 and Magnus was captured. He was then dethroned, blinded, castrated and had one legg cut off. After this he was known as Magnus the Blind.

Magnus then spent some time as a monk. Harald Gille was killed in 1136 by Sigurd Slembedjakn , another royal pretender. To gain support, Sigurd Slembedjakn had Magnus removed from the abbey. From then on, Magnus made common cause with Sigurd to regain his kingdom. They decided to split up. Magnus went to Eastern Norway, where he had most popular support, but was defeated at Minne by King Inge. He then fled first to Götaland and then to Denmark, where he tried to get support for his cause. An attempted invasion of Norway by King Erik Emune of Denmark failed miserably. Magnus then rejoined Sigurd's men, but they continued to have little support in Norway. After some time spent more like bandits than kings, they met King Inge and King Sigurd in a final battle at Hvaler 12th November 1139. Magnus fell during the battle, and Sigurd Slembedjakn was captured and killed.