On 7 October 1954, Hersch was elected a judge of the International Court of Justice as successor to Sir Arnold McNair who was reaching the end of his nine-year term and did not wish to stand for re-election. This ushered in the last and in some ways the most significant period of Hersch's life. Great as his reputation was by then, on the basis of his four major books, his editions of Oppenheim, editorship of the British Year Book of International Law and membership of the International Law Commission, it is to his separate and dissenting opinions as a judge of the Court that one looks for the ultimate practical implementation of his scholarship.

The ICJ was at that time regarded as the most important international tribunal in the world. Indeed, with the possible exception of the European Court and the European Commission of Human Rights, in its limited sphere, it had no competitor. It was described in Chapter xiv of the Charter of the United Nations as ‘the principal judicial organ’ of that body. Its jurisdiction extended to any dispute between States that they might agree, in one or another form, to submit to it. It did not have, and still has not, any jurisdiction over cases initiated by individuals. But it also possessed, and still possesses, the competence to give advisory opinions at the request of various organs of the UN and its specialised agencies – requests most often made by the General Assembly or the Security Council.