This article examines the mixed gender justice outcomes of the International Criminal Court’s (ICC) first case, The Prosecutor v. Thomas Lubanga Dyilo, and argues that they were influenced by competing institutions: older gender-biased norms of international law and new formal gender justice rules of the ICC’s Rome Statute. Using a feminist institutionalist framework, the article suggests that formal and informal institutions work together in multiple ways to produce different outcomes, and that in understanding the operation of informal institutions, it is as important to search for silences and inaction, as it is to identify articulation and action.