Most information about Magnus is gleaned from Norse sagas and chronicles, which will only begin to appear during the twelfth century.

The most important sources still available are the Norwegian chronicles Historia de Antiquitate Regum Norwagiensium by Theodoric the Monk and the anonymous Ágrip af Nóregskonungasögum (or simply Ágrip) from the 1180s and the Icelandic sagas Heimskringla (by Snorri Sturluson), Morkinskinna and Fagrskinna, which date to about the 1220s.

While the later sagas are the most detailed accounts, they are also generally considered the least reliable.

Additional information about Magnus, in particular his campaigns, is found in sources from the British Isles, which include contemporary accounts.

Although sources are unclear about the first year of Magnus Barefoot’s reign, it is apparent that the king’s focus is on the west (towards the British Isles).

Since conditions have been chaotic in Norse-dominated parts of the British Isles since the death of Thorfinn the Mighty, this provides Magnus an opportunity to intervene in local power struggles.

According to some accounts, he made his first expedition west in 1093–94 (or 1091–92), helping Scottish king Donald Bane to conquer Edinburgh and the Scottish throne and possibly gaining control of the Southern Isles (Suðreyjar) in return.

It is unclear if this early expedition took place, since it is not directly referenced in early reliable sources or the sagas.

Haakon has been proclaimed king in the Uplands and at the Øyrating, the thing of Trøndelag (in central Norway).

According to Førsund, Haakon took control of the entire portion of the kingdom once held by his father (also including the Frostating—the thing of Hålogaland in northern Norway—and the Gulating—the thing of western Norway).

Haakon has secured support by relieving farmers of taxes and duties (including taxes dating back to the Danish rule of Sweyn Knutsson during the early 1030s), while Magnus pursues costly policies and demands lengthy military service.

After Magnus settles at the new royal estate in Nidaros for the winter of 1094–95, Haakon also travels to the city and takes up residence at the old royal estate.

Their relationship becomes increasingly tense, culminating when Haakon sees Magnus' longships fully rigged at sea.

Haakon summons the Øyrating in response, leading Magnus to sail southwards.

Haakon attempts to intercept Magnus by traveling south to Viken by land (over the mountains of Dovrefjell), but he dies unexpectedly while hunting in February 1095.