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A canny Chagrian politician, Mas Amedda served as the Republic’s Vice Chancellor under Valorum during the Naboo crisis. When Senator Palpatine succeeded Valorum as Chancellor, Amedda retained his position under the Republic’s new leader.
Amedda was one of the few in the galaxy who knew Palpatine was a Sith Lord, and assisted the Chancellor as Speaker of the Senate, helping him manipulate Senators into granting him more and more power.
Amedda’s loyalty was rewarded. For years during the Empire's reign, he remained at Palpatine’s side as Grand Vizier of the Empire, working behind the scenes to serve Sheev's machinations and drive the creation of the Imperial stormtrooper program as well as countless strongholds and dedicated servants like Grand Moff Tarkin and Lothal's Arihnda Pryce.