The struggle to claim Ingeborg's inheritance from her murdered father later involved Norway in intermittent conflicts with Denmark for decades to come.

In 1266 he gave up the Hebrides and the Isle of Man to Scotland, in return for a large sum of silver and a yearly payment, under the Treaty of Perth, by which the Scots at the same time recognised Norwegian rule over Shetland and the Orkney Islands.

[5] Magnus also seems to have had good relations with the Swedish King Valdemar Birgersson, and in the 1260s, the border with Sweden was officially defined for the first time.

When Valdemar was deposed by his two brothers and fled to Norway in 1275, this stirred Magnus into gathering a leidang-fleet for the first and only time in his reign.

In internal politics, Magnus carried out a great effort to modernise the law-code, which gave him his epithet law-mender.

His code introduced the concept that crime is an offense against the state rather than against the individual and thus narrowed the possibilities of personal vengeance.

A specific section fixed the law of succession to the throne, in accordance with the arrangements laid down by King Håkon Håkonsson in 1260.

The nationalist-conservative historian Oscar Albert Johnsen, who headed the Institute of Medieval Texts, founded during the occupation of Norway by the Nazis, considered Magnus a weak king for giving up the Hebrides and acquiescing to the demands of the Church.

[9] Mostly considered a rather wise king, sparing the kingdom of unnecessary and unfruitful wars, while preserving stability at home and finally bringing the Icelandic Commonwealth under Norwegian control.