Mr Yillah is a Barrister and Solicitor of the High Court of Sierra Leone. He was called to the Sierra Leone Bar in 1998 and has practiced law both nationally and internationally. Domestically, he has appeared before all levels of the judiciary in Sierra Leone, including the Magistrate’s Court, High Court, Court of Appeal, and the Supreme Court.

At the international level, Mr Yillah has held various roles: he served as Associate Counsel in the Principal Defender’s Office at the Special Court for Sierra Leone, later becoming Co-Counsel at the same court. From 2005 to 2011, he served as Associate and Trial Lawyer in the Office of the Prosecutor at the International Criminal Court (ICC) in The Hague. He has also acted as Defence Counsel before the ICC in cases related to Darfur. In 2020, Mr Yillah was elected by his peers to the Executive Council of the International Criminal Court Bar Association.

Since 2015, Mr Yillah has served as Principal Defender of the Residual Special Court for Sierra Leone. In this capacity, he, along with pro bono counsel, represents all individuals convicted by the Special Court who are currently serving their sentences in Rwanda and the United Kingdom. He has also worked on a range of human rights matters, including supporting AIDS-Free World in its request for the prosecution of former Gambian President Yahya Jammeh. Mr Yillah is currently involved in efforts to obtain redress for victims of human rights violations committed by foreign troops in Sierra Leone during the civil war.

In addition, Mr Yillah is a lecturer in International Criminal Law at the Sierra Leone Law School, where he focuses on teaching victim participation and the role of the Trust Fund for Victims in delivering reparative justice at the ICC.

Mr Yillah resides in Freetown.