Last king of the Norse line in Mann, son of Olave the Black, who reigned from 1252 until his death at Castle Rushen.

Speed's account gives the two ends of the reign plainly. In 1252 Magnus, the son of Olave, came to Man and was made king, and in the following year he went to the King of Norway and stayed there a year. He had a safe conduct from the King of England in 1253, as heir of Mann and of the Islands, for himself and his family to pass through the King's possessions towards Norway and to return.

His title was contested even in the record. The Chronicle described his brother Reginald as the last of the race, which the Manx Society editor called a manifest error, adding that Magnus succeeded him and was king de facto if not de jure. The numbering of the dynasty varies with the source: one list makes him the ninth of the race of Godred Crovan, another the tenth, and a third counts twelve kings in all.

Two charters survive from the middle of the reign. In 1256, styling himself by the grace of God King of Mann and the Islands, he granted to the prior and convent of Conishead for the salvation of the souls of all his kindred, dating the deed at the abbey of Furness. In 1257 he granted to Richard, Bishop of Mann and the Isles, the charter which constituted the bishopric a feudal barony. The editor of the Manx statutes was sharp about what that meant in practice: the bishop obtained more than the privileges of a feudal prince and none of the responsibilities, and the king kept the whole expense and burden of defending the Island. The manuscript date of 1329 on the charter is an error, and the grant is more probably of 1257, when Richard dedicated the church of St Mary of Rushen.

He died in 1265 at Castle Rushen and was buried in the abbey church he had lately caused to be dedicated, leaving no child. One account has him back at Castle Rushen during the negotiations which ended in the Treaty of Perth, given no consideration by either party to them, and dying in disgust. The treaty itself, concluded at Perth in 1266 between Magnus IV of Norway and Alexander III of Scotland, is a different Magnus, and by it the Isles were surrendered to the Scots for four thousand marks in four years and a hundred marks yearly thereafter, payable in Orkney in the church of St Magnus.

The consequences ran on for forty years. In 1290 the men of the Isle of Man petitioned Edward I for his protection, describing their Island as lately laid desolate and oppressed with many enemies after the death of Magnus their king and the forcible transfer of their country to Scotland, and they attached their common seal to it. In 1293 Aufrica, calling herself kinswoman and heiress of Magnus, obtained a citation of the King of Scotland to answer her appeal in England. She made over her right to her husband Sir Simon de Montacute in 1306. His charter to Bishop Richard was confirmed by Thomas Randolph as Scottish Lord of Man, and again by Sir John Stanley on 11 July 1423.