He was the son of Gille Críst, Earl of Angus, and his second wife, Ingibiorg Ericsdottir of Caithness; Ingibiorg was the granddaughter of Orkneyar Jarl Rögnvald Kali Kolsson. Magnus was granted the Earldom of Orkney by King Haakon IV of Norway in 1236 following the death of Jon Haraldsson in 1231 despite there being several other claimants to the jarldom. (The first Magnus of Orkney was St. Magnus Erlendsson, who ruled from 1108 to about 1117.) He died three years later, and was succeeded by his son Gilbert.
- "The Angus Line (c1236-c1325)". Orkneyjar: The Heritage of the Orkney Islands. Retrieved 9 March 2021.
- Saint-Clair, Roland William (1862). "The Earldom of Caithness 871-1476". The Saint-Clairs of the Isles; Being a History of the Sea-Kings of Orkney and Their Scottish Successors of the Sirname of Sinclair. ISBN 978-1-298-50478-4.