Mattie also maintains a dynamic pro bono practice, defending and representing a variety of clients including advocacy, health, and non-profit organizations. She has litigated novel asylum and habeas corpus cases in immigration and federal court. She successfully argued a prisoner civil rights case in the Fourth Circuit, resulting in a published decision clarifying the procedure for district courts handling similar claims. She serves as a member of the Capital Area Immigration Rights Coalition Legal Advisory Committee. Prior to joining O’Melveny, Mattie clerked for the honorable Robert C. Chambers, US District Judge, Southern District of West Virginia.