In 60 BBY, upon his rise to the position of Mand'alor, Jaster Mereel, a Mandalorian soldier, established the Supercommando Codex as a set of guidelines for Mandalorian behavior. As a man with strong ethics, Mereel observed that many Mandalorians of his time had grown disillusioned with the dishonorable and excessively brutal practices that had become widespread among numerous warriors within the culture. Mereel decided to become a reformer, aiming to restore honor to the clans and revive the ancient warrior codes that had largely been forgotten over time. To achieve this, Mereel revitalized and modernized the ancient Canons of Honor followed by the Mandalorian Crusaders and Neo-Crusaders, creating a collection of several hundred commandments governing moral Mandalorian conduct, which he named the Supercommando Codex. Similar to the Canons before it, the Codex heavily incorporated the Resol'nare, the six core principles of Mandalorian culture. Through the Codex, Mereel advocated that Mandalorians who desired to fight should cease engaging in the disgraceful activities of raiders and brigands. Instead, Mandalorian warriors should conduct themselves as highly-paid soldiers and honorable mercenaries.
However, Jaster Mereel's reforms did not receive universal approval, and not all Mandalorians consented to adhere to the Supercommando Codex. The pacifist New Mandalorians rejected violence and even Mereel's reformist warrior codes, while a contingent of Mandalorian radicals united under Tor Vizsla to establish the Death Watch, a breakaway faction vehemently opposed to the Codex and the changes Mereel sought to implement. Civil war erupted between the Death Watch and an army of supercommandos loyal to Mereel, known as the True Mandalorians. Although the True Mandalorians were largely defeated due to Death Watch's devious schemes, the Death Watch was similarly decimated at the hands of Mereel's adopted son, Jango Fett. The principles of the Supercommando Codex persisted within Mandalorian society long after Mereel and his True Mandalorians had fallen. For example, the Mandalorian Protectors derived their moral teachings from the Codex and endeavored to uphold its stringent standards.
The code of Jaster Mereel was initially hinted at in the first issue of the Jango Fett: Open Seasons comic series, but the Supercommando Codex was not fully introduced to the Star Wars universe until the publication of Abel G. Peña's 2005 Star Wars Insider article, The History of the Mandalorians. The subsequent reference book, The New Essential Chronology by Daniel Wallace, briefly mentioned the Supercommando Codex and its significance in Mandalorian history. Wallace would later feature the Codex again in his work in the Galaxy at War reference book. The online role-playing guide for the Star Wars Galaxies video game, titled The Way of the Warrior: A Guide to becoming a Mandalorian Soldier, also referenced the Supercommando Codex as a potential element of a Mandalorian player's roleplaying backstory, as did The Essential Guide to Warfare by Jason Fry and Paul R. Urquhart.