In 1992, District Attorney Tierney began his career as an Assistant District Attorney at the Suffolk County District Attorney’s Office, where he served under then-District Attorney James Catterson. He worked his way up through the ranks of the office, and handled serious felony prosecutions including kidnapping, arson, violent sexual assault, and murder. District Attorney Tierney was promoted to District Court Trial Supervisor, where he supervised entry-level ADAs handling misdemeanor cases. After seven years at the Suffolk County District Attorney’s Office, District Attorney Tierney joined a local law firm in 1999, where he managed the firm’s litigation and tried cases before juries in Bronx, Queens, Nassau, and Suffolk Counties, until 2002.