Developed by Anaheim Electronics, the Javelin was based on the frame of the RGM-119 Jamesgun and the early model, and share most of its parts and system designs with the Jamesgun. The later production model, which was the one used during the Zanscare War, was built with completely original parts. In addition to various thrusters that improve its mobility over the Jamesgun in space, the Javelin was also equipped with a backpack fitted with a pair of shot lancers, inspired by similar weapons of the same name used by the Crossbone Vanguard's mobile suit. The shot lancers can be replaced by other weapons depending on the situation, resulting in variants such as the RGM-122 Javelin Mega Spear Loading Type and the RGM-122C Javelin Cannon. Although the Javelin is the best mass production MS available to the Earth Federation, it is still several decades old by the time of the Zanscare Wars, and was outclassed by late Zanscare war designs.
The RGM-122 Javelin was deployed by the Earth Federation during the first half of the second Universal Century as the replacement for the older RGM-119 Jamesgun. Their exact roll out date is unknown, but they were seen as early as U.C. 0133, fighting alongside Jamesguns and limited production F91 Gundam F91 against the forces of the Jupiter Empire. During the battle, Javelins destroyed more Empire mobile suits than Empire suits did Javelins, though it is unknown if it was because of a technological difference between the machines or a skill difference between the pilots. The Javelin would then acquit itself very well against the Empire during the counterattack with the Crossbone Vanguard and the Colonial Earth Federation Forces.