After the death of Lawrence Trask, the property of plans for the Sentinels fell to the government of the United States. Dr. Stephen Lang became the head of a federal research – Project: Armageddon – about mutants, trying to discover how mutations could provide superhuman powers. However, unbeknownst to his employers, Lang was an anti-mutant bigot who used his position to obtain access to the notes and designs of Larry Trask and Bolivar Trask, his father and creator of the original Mk I Sentinels.

Lang also went to the Council of the Chosen Ones, part of the Hellfire Club, who provided him with funding to create new Sentinels and an orbital headquarters. The Council also ignored that Lang wanted to destroy every mutant he could capture as their leader, Ned Buckman, shared Lang's prejudices. After he used one of Lang’s Sentinels in an attack on fellow Council member Sebastian Shaw, an industrialist and secretly a mutant himself, a quiet civil war broke out where Buckman and his cohorts were murdered and replaced by Shaw’s mostly mutant Inner Circle.

Surprise attacks upon the X-Men as they celebrated Christmas in Manhattan and upon Professor X while vacationing in the Bahamas led to their capture. Encased in “atmo-spheres”, the assembled mutants were taken to Project: Armageddon’s space station for analysis and eventual execution by Lang and his followers.

The Mk III Sentinels were unfortunately inferior to the previous ones because Lang worked with only incomplete notes from the Trasks and because the secret of his activities stopped him from hiring enough experts. He also made them less intelligent than the previous Sentinels to stop any possible rebellion. Notably, Lang built Sentinels that looked like the original X-Men, which became known as the X-Sentinels. The X-Sentinels confused the newer X-Men to such a degree that they were ending up thrashed, but then Wolverine discovered that these impostors were not human and ripped up the Jean Grey lookalike, exposing their true nature.

Lang's Sentinels and space station were destroyed in a battle between the Sentinels and the X-Men. Lang was lobotomized during the battle, but he had made a backup of his mind patterns that was used as a base for the mind of Master Mold and his other Sentinels.

Unknown to the X-Men, another mutant, their old enemy The Vanisher, had also been captured by Lang and was being kept prisoner aboard the satellite. The damage caused by the battle and the X-Men's escape unleashed the Vanisher, who found semi-completed Sentinel robots which he managed to take control of them, Making plans to use them to attack his old enemies, the X-Men, he next recruited fellow mutant criminals The Blob, Lorelei, and Unus the Untouchable. Chased by a few of the Vanisher's Sentinels, the trio approached the Champions, seeking asylum from the apparently continued threat of Stephen Lang's Sentinels. Eventually taking the heroes captive, the villains dropped the ruse and the Vanisher explained that he intended to use them to draw out the X-Men, as to of the CHampions, Iceman and Angel, were former members.

Darkstar then lashed out with the full potential of her powers, defeating the former Brotherhood members and apparently killing the Vanisher when her powers interfered with his attempt to teleport away. She also destroyed the Sentinels, apparently ending the last of the Mark III model.

• Atmo-Sphere: Mk III Sentinels could generate a life-support bubble to keep up to six human-sized life forms. This allowed them to transport living beings through outer space. The bubble was tough, but not indestructible.
• Energy Blast: The Mark III had an amount of different blasts, including but not limited to plasma, electrons, stunners and heat, that could be shot from their palms. They were also armed with a disintegrator that only worked with inorganic matter.
• Flight using jets built into their feet.
• Mutant Detection: The Sentinels constantly scanned all the living beings in the immediate surrounding area and were able to determine if they were humans or mutants.
• Robot: Due to its nature, the sentinels were protected by the sheer strength of the hull. They could not be affected by emotional or mental attacks, or by illusions. However, they were susceptible to attacks affecting machinery.
• X-Sentinels: The X-Sentinels were constructed to look, sound, and fight like the original X-Men, including Professor Xavier, Havok and Polaris, even to the point of imitating their originals' powers. Their disguise meant, however, that they might not have been able to employ other standard Sentinel abilities, such as their jet-powered flight.

Flight under their own power.