In 2002, the UN Secretary-General Kofi Annan appointed him as Deputy Prosecutor for the Special Court for Sierra Leone, at the level of an Assistant Secretary-General, and in 2005 promoted him to the post of Chief Prosecutor at the higher level of Under Secretary-General. Silva brought about the arrest of Charles Taylor, former President of Liberia, who was convicted of war crimes at the Hague in 2011.
De Silva was called to the bar from the Middle Temple in 1964, and was appointed a Queen's Counsel in 1984. A member of the Criminal Bar Association and the International Association of Prosecutors, he became one of the highest-profile criminal Queen's Counsel in England.