Made up of units like Torrent Company, the 501st fought in many battles, such as the Battle of Teth and the Battle of Coruscant, becoming one of the most well-known legions. In 19 BBY, the 501st became Darth Vader's personal unit as he led them during Operation: Knightfall, in which the clones helped take down the Jedi, whom they believed to be traitors to the Republic.
Following the rise of the Galactic Empire, the 501st transitioned into a stormtrooper unit, like other clone units. Given new armor and weapons, the 501st faithfully served the Empire, operating under the direct command of the Sith Lord Darth Vader. After the Battle of Kamino, the 501st became the last remnant of the Grand Army of the Republic fully composed of Jango Fett clones, as opposed to the ever-growing and more diverse Stormtrooper Corps, where the Fett clones became a minority, with the inclusion of a pool of clones from different templates and birth-born recruits, while others were not clones, but humans from an Imperial academy.
The legion fought in a number of battles during the Galactic Civil War in an effort to defeat the Rebel Alliance and bring peace to the galaxy, although the Alliance ultimately triumphed. Though the 501st was disbanded after the Battle of Endor, the Chiss Grand Admiral Thrawn resurrected the designation by forming a new 501st Legion. He also added more diverse recruits to the 501st, including non-Humans. The reconstituted 501st was stationed in the Unknown Regions, where they protected Thrawn's Empire of the Hand. Over a century later, the legion was in the service of a new Empire, by then under the rule of Emperor Roan Fel.
When Fel was deposed by Darth Krayt shortly after the conclusion of the Sith–Imperial War, the 501st Legion fell under the control of the Dark Lord of the Sith in the year 130 ABY. Seven years later, however, Fel returned from exile and joined in the war against Krayt in order to reclaim his throne. Under the leadership of General Oron Jaeger, the 501st deserted the Dark Lord's Galactic Empire in order to join the Fel faction.