Beginning in 2008, District Attorney Tierney joined the United States Attorney’s Office and served as an Assistant United States Attorney in the Long Island Criminal Division for the Eastern District of New York. There, District Attorney Tierney oversaw complex criminal investigations and prosecutions on behalf of the United States Federal Government. In this role, District Attorney Tierney served as lead counsel for all phases of jury trials for crimes including complex white-collar schemes, violent crimes, internet crimes, international narcotics enterprises, and money laundering schemes. He also handled all post-conviction phases of his criminal prosecution, including drafting appellate briefs and arguing appeals before the Second Circuit Court of Appeals.
District Attorney Tierney also coordinated with Department of Justice leaders regarding long-term international criminal prosecutions, international fugitives, and death penalty protocols, including debriefing the Attorney General on the status of MS-13 prosecutions nationwide.