In the Hversu Noregr byggdist and in Thorsteins saga Víkingssonar, the name is attributed to the mythical king Raum the Old (Raumr inn gamli).

Snorri Sturluson relates in his Heimskringla that it was ruled by the semi-legendary Swedish kings, Sigurd Hring and Ragnar Lodbrok during the 8th century.

In the 9th century, Halfdan the Black, the father of King Harald Fairhair, subdued the area by defeating and killing the previous ruler, Sigtryg, in battle.

However, it was forcibly conquered by Harald Fairhair who spent a summer there, to ensure that Romerike was integrated into his newly-created kingdom of Norway.

The centre of the kingdom and the earliest settlements were at Sand, between Jessheim and Garder, where the soil was easiest to cultivate and the surrounding forests were rich in game animals.