In the distant future, year 3037, the Glorft created Megas as a Prototype weapons to be used on the Earthers and end the war once and for all. However, a woman named Kiva Andru stole the prototype and made modifications to it, renaming it M.E.G.A.S (Mechanized Earth Guard Attack System). What Kiva intended to do with Megas was to use its time drive to travel back in time to the point of the Glorft-Human War where a critical turning point, the battle of the last stand, happened and use Megas to turn the war's favor towards Earth. However that all changed when the Glorft invaded the base Kiva was in, having to put her plan ahead of schedule.
Ultimately, the Glorft attacked her while she was trying to transfer into Megas from her own Mech in order to take it back in time to the year 3035 to use it in the battle of the last stand to defeat the Glorft. As a result, the cockpit of Megas was damaged and the tube that Kiva used to dock into Megas was torn from its position, forcing Kiva back into her own mech. She already activated the time flux unit, but the attack caused the objective marker to malfunction, sending Megas back all the way into the 1930's and leaving Kiva and the Glorft, who followed in after her, stranded in time.
Then, in 2004, Coop found Megas in a Jersey City junkyard and it was in very bad shape with its head being blown up from the earlier attack. He then proceeds to carry out heavy modifications to its paint job and frame, then puts a car to replace the destroyed head. He also destroyed the time flux unit that Megas had inside, not knowing what it was and what it does. He then dubs it XLR (eXtra Large Robot). The modifications he carried out effectively made it impossible for anyone but him to pilot Megas, as Kiva noted after Coop defeated her.
After Coop defeated the Glorft for the first time, then Kiva agreed to train him to pilot Megas, for the coming battles against the Glorft. She is also trying to fix the time flux in order to get back to the future.
Coop would go on to have many adventures with Megas, going to different planets, fighting monsters, other giant robots, and so on.
It was finally destroyed by Evil Coop from an alternate dimension in battle, after which it was replaced by its counterpart Megas from that same dimension.