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