 Jón Magnússon was the son of Magnus III of Orkney, and succeeded his brother to the Earldom of Orkney and Earldom of Caithness in 1284.He was a signatory of the Treaty of Birgham/Salisbury in 1290, in which Margaret, Maid of Norway, was betrothed to Edward of Carnarfon.On 5 August 1296 he swore fealty to Edward I of England at Murkle, in Caithness.