How do humanitarian non-governmental organisations (NGOs) define and institutionalise global accountability standards? This article process-traces the case of the Humanitarian Accountability Partnership-International (HAP-I), a voluntary, self-regulatory collective accountability initiative, to investigate the processes through which NGOs define collective rules, standards, and practices for accountability. This article shows the limitations of traditional representative and principal-agent models of NGO accountability when applied to the global inter-organisational realm and argues that mutual accountability better conceptualises these relationships. In this important case, the article finds that transnational coordination of NGO accountability practices results from social learning that generates a global accountability community (GAC) constituted by mutual engagement, joint enterprise, and a shared repertoire of practices. Data from the process tracing shows a collaborative not hierarchical or coercive relationship between NGOs and states, where salient donors changed their understandings and practices of accountability during the process of developing the HAP-I benchmarks. Thus, GACs both regulate the behaviour of members and constitute their social identities, interests, and practices.