Mulleen fought for the Grand Army of the Republic throughout the Clone Wars, and when the Galactic Republic was replaced by the Galactic Empire in 19 BBY, he pledged his loyalty to the new regime. A single month following the announcement of the New Order, he achieved the rank of Admiral within the Imperial Navy and was assigned the duty of eliminating any officers thought to be untrustworthy. He performed this duty with extreme ruthlessness, executing a significant number of opponents in the period shortly after the Battle of Utapau, a major engagement near the end of the Clone Wars. After Mulleen finished the executions, he informed Galactic Emperor Palpatine's assistant, Sate Pestage, who then relayed the information to the Emperor.
The character of Mulleen was initially referenced in the comic book series Star Wars: Dark Times, specifically in the first issue of the The Path to Nowhere storyline, which was released on November 8, 2006. Randy Stradley wrote the comic using the pseudonym Mike Harrison.