Sustainable Development Goal Three is rightly ambitious, but achieving it will require doing global health differently. Among other things, progressive civil society organisations will need to be recognised and supported as vital partners in achieving the necessary transformations. We argue, using illustrative examples, that a robust civil society can fulfill eight essential global health functions. These include producing compelling moral arguments for action, building coalitions beyond the health sector, introducing novel policy alternatives, enhancing the legitimacy of global health initiatives and institutions, strengthening systems for health, enhancing accountability systems, mitigating the commercial determinants of health and ensuring rights-based approaches. Given that civil society activism has catalyzed tremendous progress in global health, there is a need to invest in and support it as a global public good to ensure that the 2030 Agenda for Sustainable Development can be realised.