In 1991, he was appointed as a Judge of the International Court of Justice in The Hague, becoming its vice-president in 1997. He presided as vice-president over several important cases before the court, including a case on the illegality of the use and threatened use of nuclear weapons. From 2000 to 2002 he served the ICJ as an ad hoc judge. Weeramantry served on the Legal and Human Rights Advisory Board of the Genetics Policy Institute and the president of the International Association of Lawyers Against Nuclear Arms. In addition C.G. Weeramantry was a councillor of the World Future Council, and the honorary patron and international advisor of the Centre for International Sustainable Development Law.