John is a South African barrister, practising international law and international criminal law in the Netherlands. For thirty years he was professor of law at the University of the Witwatersrand, Johannesburg, where he directed the Centre for Applied Legal Studies, a unit that engaged in human rights research, advocacy and litigation. He participated in the drafting of the Bill of Rights of the 1996 South African Constitution. From 1995 to 1997 he was Director of the Lauterpacht Research Centre for International Law, Cambridge, and from 1998 to 2006 he was professor of international law at the University of Leiden, where he currenty lectures on a part time basis as Professor Emeritus of Public International Law.
John was a member of the UN International Law Commission for fifteen years and was Special Rapporteur on Diplomatic Protection to this body. In 2001 he was appointed as Chair of the UN Human Rights Inquiry Commission to Investigate Violations of Human Rights and Humanitarian Law in the Occupied Palestinian Territory and from 2001 to 2008 he was UN Special Rapporteur on the Situation of Human Rights in the Occupied Palestinian Territory. Since 2000 he has served intermittently as Judge ad hoc of the International Court of Justice. John is presently sitting as a Judge ad hoc at the International Court of Justice in the case of Costa Rica v Nicaragua.
John has held visiting professorships at the universities of Princeton, Duke, Berkeley, Pennsylvania, New South Wales and Pretoria. He is a member of the Institut de droit international and an Honorary Member of the American Society of International Law (2008). Six South African universities have conferred honorary degrees on him.