Magnus was a member of Althingi for his constituency in North west Iceland from 1916 till the day of his death in 1937.

He served as prime minister of Iceland for 15 days, from 23 June to 8 July 1926 following the death of Jón Magnússon.

Magnus is the shortest serving prime minister in Icelandic history.

Magnus was a member of the now defunct Conservative Party (Íhaldsflokkurinn).

He was the Minister of Industrial Affairs in the presiding Government of Jón Magnússon from 1924 to 1927.