MechaGodzilla first appeared in a pseudo-flesh outer covering, masquerading as the real Godzilla during attacks against Japan. The Simians gave MechaGodzilla a flamethrower located in his mouth in order to mimic Godzilla's signature Atomic Breath, but they didn't bother replicating Godzilla's unique roar. Godzilla's ally, Anguirus, wasn't fooled by the impostor, and attacked. The battle went badly for Anguirus, whose lower jaw was broken in the process, forcing the monster to retreat. But the battle tipped humanity off to the charade.
Soon the true Godzilla appeared and burned off his rival's disguise. Interestingly, after losing his disguise, MechaGodzilla's fingers lost all mobility (probably due to suit limitations). The revealed MechaGodzilla then overwhelmed Godzilla, but was called back for repairs because of the damage Godzilla inflicted on it.
MechaGodzilla was later re-launched when the Black Hole aliens found where King Caesar was sleeping, and hoped to kill him before he could awaken. MechaGodzilla proved unsuccessful and had to fight King Caesar, whom initially held the upper-hand because of his ability to reflect MechaGodzilla's eye-beams back at it, but the robotic terror then pulverized him in a fist fight. Godzilla then entered the battle and briefly managed to turn the tide, until MechaGodzilla unleashed its entire arsenal on Godzilla and King Caesar. MechaGodzilla would then proceed to attack from the air, prompting Godzilla to deploy a last ditch attack: he generated a large magnetic field (how he did this isn't explained, it's implied it might have been from lightening bolts that struck him earlier in the movie). MechaGodzilla attempted to fly away from Godzilla, but the magnetic field proved too strong for the robot, and it ended up getting stuck to Godzilla. Godzilla's added weight made escape impossible for MechaGodzilla. King Caesar then headbutted MechaGodzilla several times, after which Godzilla twisted around MechaGodzilla's head (oddly, MechaGodzilla earlier displayed the ability to spin its head around easily), and then broke it off. MechaGodzilla exploded, sending millions of pieces into the ocean. But the Simians still hadn't given up.
The Simians rebuilt their dreadnought for another try one year later. Having learned the value of teamwork firsthand, the Simians called in an old debt to pair MechaGodzilla with the aquatic dinosaur Titanosaurus. First Titanosaurus fought Godzilla alone but failed.
Later, MechaGodzilla was rebuilt by mad scientist Mafune, believing that MechaGodzilla is the answer to his revenge against his own species: humans, as the other biologists thought he was "crazy". Mafune tried to explain the procedures of the existence of Titanosaurus. Mafune complained that he doesn't have to obey the Simeon aliens, believing that Titanosaurus will fight MechaGodzilla, also thinking Titanosaurus has brains, thus is much more powerful than the machine MechaGodzilla. The two monsters team up to fight Godzilla, which only made Mafune angry that his plans didn't go well. Later, his daughter told him that he's giving the Earth to the aliens, and soon, his daughter Katsura was rebuilt as a cyborg to control the monsters.
The aliens promised Mafune, if he uses the power of Titanosaurus well to aid MechaGodzilla, then the aliens will build a new home much more complicated in technology built from the ruins of Tokyo, more than any other home mankind had designed, for only him and Katsura to live happily ever after. Soon, Mafune had no idea how supersonic waves affected Titanosaurus, his only, and greatest weakness. While Godzilla didn't have to fight Titanosaurus, he had still a very hard situation fighting MechaGodzilla. Godzilla couldn't even stop it by pulling off his head, where MechaGodzilla fired a giant Control antennae beam at him, nearly killing Godzilla.
Mafune tried to rescue Katsura from harm from the film's protagonist: an Interpol agent who fell in love with the human of Katsura. As Katsura was now a evil slave of the aliens, she commited suicuide by using a gun to kill herself (in the American version, one of the Interpol agents shot her), and then, Mafune was accidentally hit by a bullet through his chest, crying out to reach Katsura, as he was dying. While the Interpol agent tells the dying Katsura robot how much it doesn't matter if she is not human, he still loved her, and none of the whole disaster to destroy the civilization on Earth is not her fault, but the alien's and Doctor Mafune's. Katsura, who was the only controlling device of MechaGodzilla, was dead, soon, the controller on the antennae on MechaGodzilla, no longer worked. In fact, MechaGodzilla lost all of its powers and was paralyzed permanently, so this leads to MechaGodzilla having no hope to defeat Godzilla. Godzilla wastes no time, and hurls MechaGodzilla to the ground and destroys all of its body with an atomic-breath attack. Titanosaurus was defeated as well soon by falling into the water.
This time, there were some modifications to it, including moving MechaGodzilla's brain down into his neck so he could function unimpaired with his head completely removed. Godzilla's perseverance combined with the timely self-sacrifice of MechaGodzilla's operator brought the machine down for good. The King of the Monsters buried MechaGodzilla's shattered form deep underground to prevent another repair job, and since this was the final film of the original Godzilla series, he was successful.
The Showa MechaGodzilla has armor constructed out of a mysterious alloy known as "Space Titanium," which actually proved to not be particularly durable, as a brief fight with Godzilla caused enough damage for it to need to be called back for repairs. It is also equipped with a staggering amount of firepower.
MechaGodzilla is capable of firing a rainbow colored optical laser that seems to be equal in power to Godzilla's atomic breath- its head can rotate 360 degrees, allowing the robotic dinosaur to shoot down an opponent that might attempt to approach from behind. Spinning its head at high speed allows MechaGodzilla to raise a forcefield. When disguised as Godzilla, MechaGodzilla could fire a heat ray from its mouth. Curiously, after the disguise was destroyed, MechaGodzilla never used the heat ray again. When MechaGodzilla had its head removed in Terror of MechaGodzilla by Godzilla, it could fire a small concentrated beam from a small antenna.
Showa MechaGodzilla is also equipped with powerful missiles in its fingers, knees, mouth, and even its toes. These missiles can explode on contact with a target, or they may lodge in the flesh of an enemy like darts. Located under a hatch in MechaGodzilla's chest is a weapon that fires a jagged orange energy beam with enough slicing power to cut a mountain in half. Showa MechaGodzilla is no slouch in close combat either, as evidenced in its combat against King Caesar, though it couldn't beat Godzilla in this area. Finally, MechaGodzilla can fly via rockets in its feet able to reach a speed of Mach 5.
The Showa MechaGodzilla appears in the video games "Godzilla: Monster of Monsters" for the Nintendo Entertainment System. In which it is one of the strongest monsters fought by Godzilla and Mothra, possessing the most health baring King Ghidorah. It attacks with punches, kicks, and a laser from its eyes. It's attacks are strong so it could prove difficult for Godzilla, but Mothra can beat it easily by flying right under its chin, where none of its attacks can hit her.
It appears as the second boss in the the North American version of "Super Godzilla" for the Super Nintendo Entertainment System. Though the Japanese version had MechaGodzilla 2, since the 1993 film had not appeared in America yet, the localization team of game designers for "Super Godzilla" changed the character to the more-familiar Shōwa MechaGodzilla for the American version. Despite the change in appearance, it overall the same fight as in the Japanese version, with some minor differences. First, as a reference to the movie, it first appears disguised as Godzilla, though the disguise disappears after one hit. Other then that, the only gameplay difference is that the MechaGodzilla 1 has its finger missiles in place of MechaGodzilla 2's Mega Buster, which do less damage. "Godzilla: Destroy All Monsters" for the Super Nintendo was a canceled game that would have featured the Shōwa MechaGodzilla. It appears "Godzilla" for TTI Duo, "Godzilla Generations" as a playable character, and in "Godzilla Generations: Maximum Impact" as a boss for the Sega Dreamcast.
Sadly, the mega-mech was scrapped from Godzilla: Save the Earth.
Finally, MechaGodzilla had a playable appearance in the Wii version on Godzilla: Unleashed making it the first Atari/Pipeworks Godzilla game to feature the Showa version of the menacing mech. It's also the first and only Godzilla game to feature all the MechaGodzilla's (excluding the MechaGodzilla that appeared in Godzilla vs. MechaGodzilla) in one game. In this game, the alien creation can spew a flamethrower from its nose and create a force field that reflects all projectiles thrown at it. Both he and MechaGodzilla are the only characters in the game who can knock out a health cell with a beam.
- The original MechaGodzilla is the only one to be referred to by numerics within the movies themselves. When it is rebuilt in its second appearance, the "MG" emblazoned on its arm has a "2" added to it, thus giving it the name "MechaGodzilla 2". It is still always referred to as simply "MechaGodzilla"; none of the versions are actually given numbers by the characters in the movies, since none of them share the same continuity.
- The Showa MechaGodzilla is the only MechaGodzilla that was created or controlled by aliens. MechaGodzilla 2 and Kiryu were both human-made weapons.
- The MechaGodzilla from Godzilla: Unleashed has a smaller head than the counterpart from the movie.