Although researchers argue for the importance of involving the public in developing health policy, there has been little focus on central research questions - such as what techniques of public participation work, in what circumstances, and why. This paper presents a realist synthesis which identifies and explains the underlying mechanisms and specific contextual factors that lead to effective public participation in health policy and planning. Peer-reviewed, English language literature was searched, which resulted in 77 articles for review and synthesis. This synthesis uncovered the underlying mechanism of 'political commitment' that generates public participation effectiveness. The other three possible underlying mechanisms, namely: 'partnership synergy', 'inclusiveness' and 'deliberativeness', were found to potentially provide further explanation on public participation effectiveness for health policy and planning. The findings of this review provide evidence that can be useful to health practitioners and decision-makers to actively involve the public when drafting public health policies and programs and, more importantly, guide them in deciding which strategies to best employ for which contexts.