[1] He is the second longest-reigning monarch in Swedish history after the current king Carl XVI Gustaf, who surpassed Magnus in 2018.

King Magnus took advantage of his neighbour's distress, redeeming the pawn for the eastern Danish provinces for a huge amount of silver, and thus became ruler also of Scania.

In 1335, Magnus married Blanche, daughter of John I, Marquis of Namur, and Marie of Artois, a descendant of Louis VIII of France.

Opposition to Magnus' rule in Norway led to a settlement between the king and the Norwegian nobility at Varberg on 15 August 1343.

Because of the increase in taxes to pay for the acquisition of the Scanian province, some Swedish nobles supported by the Church attempted to oust Magnus, setting up his elder son Erik as king.

[9] On 12 August 1323, Magnus concluded the first treaty between Sweden and Novgorod (represented by Grand Prince Yury of Moscow) at Nöteborg (Orekhov) where Lake Ladoga empties into the Neva River.

In 1337, religious strife between Orthodox Karelians and the Swedes led to a Swedish attack on the town of Korela (Keksholm, Priozersk) and Viborg (Viipuri in Finnish, Vyborg in Russian), in which the Novgorodian and Ladogan merchants there were slaughtered.

While he spent much of 1351 trying to drum up support for further crusading action among the German cities in the Baltic States, he never returned to attack Novgorod.

In 1363, members of the Swedish Council of Aristocracy, led by Bo Jonsson Grip, arrived at the court of Mecklenburg.

At the nobles' request, Albert of Mecklenburg launched an invasion of Sweden supported by several German dukes and counts.

In spite of his many formal expansions his rule was considered a period of decline both for the Swedish royal power and for Sweden as a whole.

Magnus's young favourite courtier was Bengt Algotsson, whom he elevated to Duke of Finland and Halland, as well as Viceroy of the province of Scania.

Because homosexuality was a mortal sin and vehemently scorned at that time, rumours about the king's alleged love relationship with Algotsson, and other erotic escapades, were spread by his enemies, particularly by some noblemen who referred to mystical visions of St. Bridget.

[20] Russians drew up an allegedly autobiographical account known as the Testament of Magnus (Rukopisanie Magnusha) which was inserted into the Russian Sofia First Chronicle, composed in Novgorod; it claimed that Magnus did not, in fact, drown at sea, but saw the errors of his ways and converted to Orthodoxy, becoming a monk in a Novgorodian monastery in Karelia.