Mechagodzilla was created by the Simeons, an advanced alien civilization whose planet was being sucked closer into a black hole. The Simeons wanted to establish a civilization on Earth, so their society could escape the black hole. The Simeons had studied Godzilla for several years and wanted to create a super-weapon based on him. Their final product was Mechagodzilla. Mechagodzilla originally had "skin" over him, which made him look exactly like the real Godzilla. While Mechagodzilla was attacking a power plant, the real Godzilla showed up and attacked Mechagodzilla. During the battle, the leader of the Simeons burned the skin off and revealed the space titanium armored Mechagodzilla. Mechagodzilla then easily defeated Godzilla because it was able to fire missiles at Godzilla without the synthetic skin on it.

Godzilla later teamed up with King Seesar, an ancient mythological creature, and destroyed Mechagodzilla. The Simeons snatched the remains away, and rebuilt an even more powerful Mechagodzilla. The Simeons teamed up with Dr. Mafune, an old scientist who wanted revenge on the Earth. Dr. Mafune discovered a dinosaur, named Titanosaurus, and controlled it with a device he made. Titanosaurus and Mechagodzilla served together as allies. Mafune suggested to the Simeon leader earlier that Mechagodzilla would be a perfect weapon if it had living brain cells. The Simeons later implanted a Mechagodzilla control-device inside of Mafune's daughter, Katsura. While fighting a winning battle against Godzilla, Katsura committed suicide (in the uncut version of Terror of Mechagodzilla), and Mechagodzilla became a sitting duck. Godzilla then easily destroyed Mechagodzilla and the escaping Simeons, and later killed Titanosaurus. Godzilla saved the Earth again!

The United Nations Godzilla Countermeasures Center (also known as "G-Force") salvaged the remains of Mecha-King Ghidorah, a cyborg constructed in the 23rd century, in order to study the technology to create a super weapon to fight Godzilla. Their first weapon, a flying ship called Garuda, was considered a failure, but their second weapon, Mechagodzilla, was a great success. Mechagodzilla was created in order to kill Godzilla for good. In Godzilla and Mechagodzilla's first confrontation, Mechagodzilla knocked Godzilla on the ground, and shocked him using the shock anchor and cables. Godzilla, though, backfired at Mechagodzilla through the cables, and severely damaged it. G-Force then created a new weapon Mechagodzilla, called the "Godzilla Crusher." The G-Crusher was a weapon that would destroy Godzilla's second brain, located on his spine.

Later on, Mechagodzilla met up with Fire Rodan, and nearly killed him. Godzilla then showed up, and almost defeated Mechagodzilla. Mechagodzilla and Garuda then were placed together to form Super Mechagodzilla. Super Mechagodzilla then used the G-Crusher weapon on Godzilla, and killed him. Fire Rodan, who was almost dead, flew over to Godzilla and sacraficed his radiation to save him. Godzilla then came back to life, more powerful and pissed off, and easily destroyed Mechagodzilla. Mechagodzilla was the closest weapon to kill of Godzilla for good. G-Force later developed another anti-Godzilla super weapon called MOGUERA, but its purpose then became to fight Spacegodzilla.