Eriksson was son of the Swedish duke, Erik Magnusson, and Ingebjørg,
daughter of the Norwegian king Håkon Magnusson.
He became the first joint king of Norway and Sweden – at the early age
When Magnus came of age, the citizens of Skåne and Blekinge in southern Sweden rebelled against their German rulers. They wanted Magnus as their king. Their wish was granted when Magnus paid 34 000 marks silver to redeem the mortgages held by the Germans.
The union treaty between Norway and Sweden ruled that the two nations should be treated equally. In Norway there was a lot of unrest in the 1330s. This may have been due to dissatisfaction with the king's rule and with the fact that Norwegian state revenue was being used to pay for the Skåne transaction. A settlement was eventually reached where Magnus had to yield to the chieftains opposing him in Norway and hand over power to his son Håkon as soon as Håkon came of age. In 1344 he was also forced to appoint his son Erik as joint king with himself in Sweden. In the period that followed, he faced a strong chieftain opposition in Sweden, an opposition led by his son Erik. When Erik died, the nobility swore allegiance to a new king, Albrecht of Mecklenburg, a nephew of Magnus. In attempting to reclaim the Swedish throne Magnus was taken captive. He was released only after formally renouncing the Swedish throne. Magnus sought refuge with his son Håkon in Norway. He died in a shipwreck off the coast of Bømlo, an island in southwestern Norway, in 1374.
During his reign the Black Death struck the Nordic nations. The death toll was chilling, and led to a significant decline in the monarchy's revenue at a time when the state was in desperate need of money.