The weapon was forged by the Huldra Brothers Brok and Sindri, back when they were relatively unknown blacksmiths, eager to make a name for themselves. Presenting the hammer to Thor, it quickly became the deadliest and favourite weapon of the Aesir, ensuring their dominance and allowing the Thunder God to massacre scores of Jötnar with devastating ease.
After seeing the destruction wrought on giants with the very weapon they created, the two brothers suffered a profound sense of guilt. As a way to try and make up for their mistake, they then forged the Leviathan axe, a weapon capable of rivaling even Mjölnir and gave it to the last guardian of the giants left in Midgard; Laufey.
The hammer is extremely powerful, capable of killing most beings with one hit and channeling a vast amount of electricity.
- Noted as the hammer's only flaw, the handle was made too short due to a fly biting Brok on one of his eyes during the forging of the hammer.
- Despite possibly being the greatest weapon in all of the Nine Realms, only rivaled by the Leviathan and Surtr's sword, Odin is capable of making certain things indestructible even by it, as according to Mimir the tree he is imprisoned in cannot be destroyed even by Mjölnir. He also comments that even the hammer can't destroy the thick ice caused by Thamur's dying breath.
- In Norse mythology, Thor has a pair of magic gauntlets and a belt that he always wore when wielding the hammer. The belt, named Megingjörð ("power-belt"), doubled Thor's already divine strength which allowed him to use Mjölnir to even greater efficiency. The gauntlets were named Járngreipr ("iron grippers") and are needed to be worn in order to wield the hammer properly, due to its short handle.
- Járngreipr and Megingjörð were not actually required to be worn in order for Thor to wield the hammer, contrary to popular belief, as has been noted to be able to wield it effectively without them. The most prominent example of this being when he slaughtered Þrymr and his kin while disguised as a bride.
- Though Brok and Sindri express guilt for all the destruction brought about by Thor's use of their hammer, they still acknowledge it as their greatest work, albeit bitterly.
- The hammer appeared before in God of War: Ascension's Multiplayer as an add-on and possible nod to the future of the franchise. This, however, is the Canon version of the hammer.
- In Norse mythology, Magni and Modi were said to have inherited the hammer together, as it was so heavy it required both of them to wield it effectively. This does not appear to be the case in God of War, however, as they are in open competition for the hammer and both Magni and Modi were killed by Kratos and his son Atreus, leaving Thor's daughter Thrud as the only possible inheritor.
- Contrary to popular depiction/belief; Mjolnir is not the source of Thor's lightning. That power is innate within in him as a storm/thunder God. The hammer is merely a weapon built to withstand the God himself as he was infamous for breaking and/or losing all other weapons that were forged for him. The Prose Edda describes the hammer as being unbreakable so it would never fail under Thor's immense strength, never missing the target it was thrown at, always returning to Thor's hand no matter how far he threw it and being of a convenient size so it was easy to carry around. There is no mention of the weapon being the origin of the Thunder God's lightning.
- Interestingly in Norse Mythology, it is implied that Loki is strong enough to wield Mjolnir. When Thor first awakens to find his hammer stolen (by Þrymr the King of the Jotnar), his first instinct is to blame the trickster, only to then realise that even Loki would not dare do something so foolish as to steal his hammer. Note; the Thunder God did not dismiss the notion on account of Loki's physical strength.