The Martens Clause continues to provide resources for a free-standing norm of customary law prohibiting acts of genocide that are free from many of the restrictions concerning, for example, protected groups contained in the original 1948 Genocide Convention's definition. This article addresses post-war developments of the Martens Clause and the codification of crimes against humanity applicable to acts of genocide. It suggests an alternative way of examining how the idea of humanity originated from the Nuremberg and post-Nuremberg developments. We also explore the historical developments of the 1948 Genocide Convention, and its application within ad hoc tribunals that have adopted a narrow definition and application. Finally, we conclude that through an expansive and sympathetic judicial interpretation and legislative reception, the Martens Clause has operated as one of the key milestones along the path that culminated in the international criminalisation of genocide.