Effective, accountable and transparent institutions, that engage in inclusive and participatory governance, are crucial for the sustainability of global development investments. However, there is a debate about whether effective approaches to improving governance processes operate from the bottom up (e.g. by enabling citizens to hold service providers accountable) or the top down (by enabling service providers to be held accountable by the State). This paper systematically reviews participation and accountability mechanisms in a range of sectors, drawing on principles of realist evaluation to develop and test middle-range theory using framework synthesis and statistical meta-analysis. We show that interventions promoting citizen engagement through participatory priority setting or accountability mechanisms are often effective in stimulating active citizen engagement in service delivery and realising improvements in access to services, where they facilitate direct engagement between service users and front-line service providers, such as in health care. However, citizen engagement interventions alone are not effective where services are accessed independently of service provider staff, for example road infrastructure. Interventions promoting participation by increasing citizens’ pressures on politicians to hold providers to account are also not usually able to influence service delivery.