Information on Guthrum is scarce outside of what the Anglo-Saxon Chronicle, Aethelweard's Chronicle, and Asser's Life of Alfred mentions his wars and the treaty of Wedmore. Anglo-Saxon Chronicle writers, Aethelweard's Chronicle and numerous works of the twelfth century mention his death in 890. About the date of his birth, no trustworthy source give it. This is not suprising since date of births are only recorded for Rollo, King Alfred, and Harold Fairhair of all the Northern leaders or kings.

However, fifteenth and sixteenth century Chronicles (Raphael Holinshed and John Brompton) gave unverifiable hints that Guthrum was the son of a man named Ring, a Danish minor king ruling a series of islands between Norway and Denmark, whose daughter became the mother of Hararld Fairhair (858-935). It appears that both derived their information from sources of the Saga of Harald Fairhair by the thirtheen century Islandic poet Sturlinson. This Gothorm (the Norse form of Guthrum) was a young teen at the time of the kidnapping of his sister by Harald's father. Being between twelve and fourtheen in 858 may indicate that he was born around 845. Are Brompton and Holinshed right in their association, it is impossible to say? I am uncertain since there was at least three sea-kings in the 870s named Fire Worm (Guthrum in Anglo-Saxon, Gorm in Danish, and Gothorm in Norse, and Gurmund in Swedish). 845 as the year of his birth is however highly plausible for man to be the leading chief in a coalition of thirty war bands.

Cf.