Origin[vague] Odin used the Uru he had been given by the Dwarves of Nidavellir when he saved their realm from the Trolls. The Trolls forged the hammer. Eitri, Brokk and Burri, the blacksmiths, used lots of heat to make it. It took seven weeks to make. The process of the star almost destroyed Midgard, which means it may have been made 65 million of years ago and have wiped out the dinosaurs.
When Odin tried to wield Mjolnir, he couldn't due to God Tempest still controlling its power. The hammer went out of control and destroyed Asgard.
Much later, the hammer was given to Odin's son, Thor. While he could not lift it at first, he soon proved worthy after only a few trials. The hammer was unbreakable and able to survive anything - even Ragnarök.