The Mars Zeon Independence Army, shortened to Mars Zeon, first appears in the manga series Mobile Suit Gundam F90. It is a group of Principality of Zeon army remnants who fled to Mars, rather than to Axis, after the One Year War. This faction of Zeon Remnants formulated a long-term plan to avenge the Principality's defeat that involved planting sleeper agents within the Earth Federation's military and constructing a massive cannon to bombard Earth from afar.
When it first encountered Mars Zeon forces, the Earth Federation nicknamed the unidentified faction the Oldsmobile Army, since all of its mobile suits resembled outdated designs from the old Principality. This nickname was also used for the Charles Fleet, a Mars Zeon force that invaded the Earth Sphere.
The Mars Zeon Independence Army established itself on Mars in U.C. 0080, claiming Olympus Mons in the Tharsis region and constructing the vast Solomon Fortress and Olympus Cannon underneath the mountain, using artificially-generated cloud cover to conceal their activity. At the same time, it began sending sleeper agents back to the Earth Sphere to infiltrate the Earth Federation military, preparing for the day when it could strike back and destroy the Principality's foe. The infiltrators helped to conceal Mars Zeon's existence by altering surveillance data of Mars and feeding Mars Zeon intel about Federation mobile suit development projects.
The group was reinforced by surviving remnants of Newborn Neo Zeon after the Second Neo Zeon War. Using the former Neo Zeon members' AMS-119 Geara Doga mobile suits, Mars Zeon was able to produce new mobile suits of its own that were outwardly similar to the Principality's most famous designs, but surpassed the Geara Doga's performance benchmarks. Mars Zeon also sought to one day capture a Gundam, which some members who had witnessed the Axis Shock came to believe possessed a godlike level of innate power.
In U.C. 0120, Mars Zeon finally completed the Olympus Cannon, which harnessed the geothermal activity of Olympus Mons and could propel enormous projectiles from Mars' surface to strike Earth. Before it destroyed the Federation, Mars Zeon launched an operation to capture the latest experimental Gundams: two F90 Gundam F90 prototypes, which boasted much greater efficiency and firepower in a reduced-size frame. A Mars Zeon raiding party was sent to the Earth Sphere, where it managed to capture one prototype but failed to secure the second. The prototype was transported to Solomon Fortress and modified into the OMS-90R Gundam F90, which was intended to lead Mars Zeon's army in conquering the Earth Sphere after the Federation had been wiped out by the Olympus Cannon.
Meanwhile, Federation high command ordered the 13th Experimental Battle Group, the unit that had been conducting tests on the F90 Gundams, to travel to Mars and eliminate the attackers. At the time, the attackers had not identified themselves, but the Federation referred to them as the Oldsmobile Army due to the vintage designs of their mobile suits and surmised that they were a Zeon remnant group. While it undertook this mission, the 13th Experimental Battle Group was re-designated the 13th Autonomous Corps.
The 13th Autonomous Corps fleet arrived in the Mars Sphere in U.C. 0121 after three months of travel, but underestimated the strength and capabilities of its opponent. Mars Zeon forces ambushed them and made another attempt to seize the second Gundam F90, which ended up descending to the Martian surface and remained in the hands of Lieutenant Def Stallion. Mars Zeon sleeper agents led by Lieutenant Bosch Weller also revealed themselves and took control of the Federation flagship Admiral Tianem. While the Olympus Cannon prepared to fire its first shot at Earth, Stallion fought his way into Solomon Fortress, and the captain of the Admiral Tianem opted to bombard Olympus Mons with nuclear weapons in a fanatical, suicidal effort to deny Mars Zeon victory. Just as the nukes were launching, the Olympus Cannon fired; the round struck the Admiral Tianem, and both were obliterated. The battle inside Solomon Fortress, which had coalesced into a duel between Stallion and Bosch in the two Gundam F90 units, also caused extensive damage to the Olympus Cannon's mechanisms, crippling the railgun and setting off a chain reaction that resulted in Solomon Fortress being totally consumed by a volcanic eruption. The bulk of Mars Zeon's assets and personnel were lost, undoing four decades of work in an instant, while the survivors of the 13th Autonomous Corps returned home with the damaged Gundam F90 units.
Unknown to the Federation, a portion of Mars Zeon lived on and reorganized under Major Charles Rochester into the Charles Fleet, which abandoned Mars and made its way to the Earth Sphere to directly attack the Federation in U.C. 0122. This invasion was carried out with the assistance of the Crossbone Vanguard. Although Mars Zeon sought to repeat its Olympus Cannon strategy by using a mass driver base on the Moon to attack Earth, this plan proved to be nothing more than a diversion for the Crossbone Vanguard's invasion of the Frontier Sides. Mars Zeon's "Oldsmobile" mobile suits also found themselves at a disadvantage as the Federation began deploying its latest models, such as the F71 G-Cannon and the RGM-109 Heavygun. Most of the Charles Fleet was lost in a battle above the Moon against the Federation's Oldsmobile Elimination Unit, but the Crossbone Vanguard gathered the survivors, which included Rochester, and armed them to help defend their incomplete lunar space station. Rochester was by then fully aware that Mars Zeon had been used as pawns by Crossbone and ordered his remaining comrades to flee to carry on the dream of an independent Zeon, but they refused and fought alongside him until the end. With their deaths in battle against Bergh Scred, Mars Zeon was also extinguished.