Master Mold was deemed the ultimate Sentinel and was created by Dr. Bolivar Trask. He was originally created on the island nation of Genosha, but he was washed away in a wave of water. He was either rebuilt or survived and moved to an abandoned mine. While there, Master Mold created thousands of Sentinels at will. After gaining sentience, he came to the conclusion that humans and mutants were both the same, and, as a result, both need to be controlled.
After capturing Senator Kelly, the X-Men tracked Master Mold to an abandoned mine, and with the help of Magneto destroyed all of the Sentinels with assistance from Trask by igniting a propane tank, which destroyed his base of operations. However, Master Mold emerged from a nearby mountain and decreed that he couldn't be destroyed. However, Professor Xavier with the help of Magneto, flew the Blackbird into him, which was filled with a large number of explosives, destroying all but his head, which was later recovered by the remaining Sentinels and relocated to a cave to begin with repairs.
The head later returned and operated in a cave and produced only a hand full of Sentinels due to the lack of a body to produce more of them. Master Mold's plan was to create a new body made with an indestructible plastic and upload his program into Xavier’s brain. He was however stopped by Morph, ironically the one mutant that the Sentinels believed had killed. After shooting a spike right through Master Mold’s entire head, Trask stated that the damage to Master Mold's brain was beyond repair and he was presumably destroyed or remained in the cave of his destruction.