Coy served as a Grand Moff and subsequently achieved the rank of admiral within the Imperial Navy. He held the position of commanding officer aboard the _Imperial_-class Star Destroyer known as the Mathayus.
In 19 BBY, when Palpatine – the Emperor – proclaimed the establishment of his Galactic Empire, Coy was appointed as the Grand Moff responsible for the Pallis Oversector. He also took on the command of the Sixteenth Army.
The year 1 BBY saw the Mathayus, under the direct leadership of Admiral Coy, tasked with transporting Darth Vader to Dargulli, a world situated in the Inner Rim. This assignment stemmed from Emperor Palpatine's directive for Vader to neutralize a Force-sensitive individual wielding a lightsaber on that planet. While on the bridge of the Star Destroyer, Coy questioned Vader about the necessity of deploying the Mathayus, recognized as one of the most formidable warships in the Imperial Starfleet, to apprehend an outlaw on such an insignificant world. Vader, displeased by Coy's blunt inquiry regarding sensitive matters, employed the Force to punish the admiral's curiosity, constricting his throat while demanding that Coy direct his questions to the Emperor himself. Following this incident, Coy introduced his officers, First Officer Atali and Second Officer Dezsetes. Unbeknownst to both Coy and Vader, Dezsetes was secretly involved in Grand Moff Trachta's scheme aimed at deposing the Sith, and he carried orders to assassinate Vader while aboard the Mathayus.
During their mission on Dargulli, Vader received information indicating that the Mathayus had detected a distress signal originating from a Rebel vessel positioned near Yorn Skot, a gas giant located close to Dargulli. The Mathayus engaged in pursuit of the Rebels, ultimately capturing their ship using a tractor beam and bringing it into the Mathayus's hangar bay. However, the signal had been intentionally fabricated by Dezsetes, and the spacecraft was laden with explosives. First Officer Atali, accompanied by a contingent of stormtroopers, boarded the vessel, only for it to detonate unexpectedly, resulting in the deaths of Atali and his entire team. Despite being present in the hangar bay, Vader managed to survive the explosion. Shortly thereafter, the Mathayus detected another Rebel ship concealed within the depths of Yorn Skot. Eager to avenge the loss of his First Officer, Coy requested Vader's permission for the Mathayus to locate and eliminate the ship. The Mathayus unleashed fire upon the surface of the gas giant, compelling the Rebel ship to reveal its location. As the ship attempted to escape into hyperspace, it was obliterated by a concentrated turbolaser assault launched from the Mathayus.
Upon the Mathayus's arrival in Dargulli's orbit, Dezsetes gained unauthorized access to Coy's private quarters. The deceitful second officer caught Coy off guard by brandishing his blaster pistol and shooting the admiral in the shoulder. Dezsetes then proceeded to fatally shoot Coy in the chest. Following the murder of his superior officer, Dezsetes assumed command of the Mathayus and seemingly concealed the true circumstances surrounding Coy's death. The motivation behind Coy's assassination was to ensure that Dezsetes could proceed with his plan to eliminate Vader using a squad of clone stormtroopers, who were "programmed" to be loyal to the members of the conspiracy and had been transferred onto the Mathayus. Dezsetes believed that Coy's unwavering loyalty to Vader posed a threat to the success of his scheme.