By various sources he is said to have been the son of Sigurd Hart, brother of Ragnhild Sigurdsdotter and the descendant of Ragnar Lodbrok.
Sources on Guthorm describes him as the son of Sigurd Hart and younger brother of Ragnhild Sigurdsdotter.
Attempts to fit the chronology of the sagas with history does not work with the additional generation between Sigurd Snake-in-the-Eye and Harald Fairhair.
Harald named one of his sons after Guthorm and subsequently made his uncle the local governor of Upland and Viken, ruling from Tønsberg.
Ragnarssona þáttr presents a near identical story to the account in Heimskringla of Sigurd Hart, his death and Ragnhild's marriage to Halfdan; as in Heimskringla Guthorm is described as Ragnhild's younger brother who is kidnapped and later rescued from Haki.
Likewise, Ragnarssona þáttr gives Guthorm's death at that time as fourteen while Heimskringla simply says he was a child, which implies an even younger age.
Guthorm is described as the regent for Harald after Halfdan the Black's death and subsequently his military commander.