Magnus was born around the end of…
October 1093 CE
Magnus was born around the end of 1073 as the only son of King Olaf III.
His mother's identity is uncertain; she is identified as Tora Arnesdatter (daughter of otherwise-unknown Arne Låge) in Morkinskinna and Fagrskinna, as Tora Joansdatter in Heimskringla, Hrokkinskinna and Hryggjarstykki and an unnamed daughter of "Ragnvald jarl" from Godøy, Sunnmøre in the genealogical text Af en gl. ætleg (commonly known as Sunnmørsættleggen).
The historical consensus has favored Tora Arnesdatter, but the other claims have also gained support.
Magnus had grown up among the hird (royal retinue) of his father in Nidaros (modern Trondheim), the de facto capital of Norway.
His father's cousin, the chieftain Tore Ingeridsson, is foster-father to Magnus.
He was apparently more similar in his youth to his warlike grandfather, King Harald Hardrada, than to his father (who bears the byname Kyrre: "the Peaceful").
According to Snorri Sturluson, Magnus was considered handsome and gifted in learning; although he was shorter in stature than his grandfather Harald, he was reportedly known as "Magnus the Tall".
Magnus' more-common byname, "Barefoot" or "Barelegs", is—according to Snorri—due to his adopting the Gaelic dress of the Irish and Scots: a short tunic, which leaves the lower legs bare.
Another version (by Danish historian Saxo Grammaticus) maintains that he acquired the nickname because he was forced to flee from a Swedish attack in his bare feet, while a third explains that he rode barefoot (like the Irish).
Due to Magnus' aggressive nature and his campaigns abroad, he also has the nickname styrjaldar-Magnús ("Warrior Magnus" or "Magnus the Strife-lover").
Norway has experienced a long period of peace during the reign of Magnus' father, Olaf.
Magnus may have been present when Olaf dies in Ranrike, Båhuslen (southeastern Norway) in September 1093 and is probably proclaimed king at the Borgarting, he thing (assembly) of the adjacent region of Viken later this month.
When Magnus becomes king, he already has a network of support among the Norwegian aristocracy.