While community mobilisation (CM) is increasingly advocated for HIV prevention, its impact on measurable outcomes has not been established. We performed a systematic review of the impact of CM within HIV prevention interventions (N = 20), on biomedical, behavioural and social outcomes. Among most at risk groups (particularly sex workers), the evidence is somewhat consistent, indicating a tendency for positive impact, with stronger results for behavioural and social outcomes than for biomedical ones. Among youth and general communities, the evidence remains inconclusive. Success appears to be enhanced by engaging groups with a strong collective identity and by simultaneously addressing the socio-political context. We suggest that the inconclusiveness of the findings reflects problems with the evidence, rather than indicating that CM is ineffective. We discuss weaknesses in the operationalization of CM, neglect of social context, and incompatibility between context-specific CM processes and the aspiration of review methodologies to provide simple, context-transcending answers.