Many social interventions have been developed with the hopes of reducing and preventing social isolation among older people (e.g., recreation, arts-based programs and social prescription). Friendly visiting programs, also known as befriending schemes, have been a mainstay in this area for decades and are largely thought to be effective at reconnecting older people (≥ 60 years of age) experiencing isolation. Research and evaluations have yet to determine, however, how and why these programs may be most successful, and under what conditions. This article presents the findings of a realist synthesis aimed at identifying the critical mechanisms and contextual factors that lead to successful outcomes in friendly visiting programs. Seven studies are synthesized to inform a friendly visiting program theory accounting for key mechanisms (e.g., provision of informal support) and underlying contexts (e.g., training of volunteers) that can be used to inform future programs. Recommendations for future research are also presented.