Social prescribing programmes (SP) are person-centred coaching schemes meant to help participants improve individual circumstances, thereby to reduce demand on health and social care. SP could be an innovative means to improve preventive and public health in the pursuit of universal financially sustainable healthcare. Given its potential, our systematic review assesses type, content, and quality of evidence available regarding SP effectiveness at the individual, system, and community levels. We examine the impact of SP on addressing loneliness, social isolation, well-being, and connectedness, as well as related concepts, which are not yet considered jointly in one study. Following PRISMA, we search: EBSCOHost (CINAHL Complete; eBook Collection; E-Journals; MEDLINE Full Text; Open Dissertations; PsycARTICLES; PsycINFO); Web of Science Core Collection; and UK National Institute for Health and Care Excellence. Excluding systematic reviews and articles without impact evaluations, we review 51 studies. Several studies do not distinguish between core concepts and/or provide information on the measures used to assess outcomes; exactly one peer-reviewed study presents a randomised controlled trial. If we wish to know the potential of social prescribing to lead to universal financially sustainable healthcare, we urge researchers and practitioners to standardise definitions and metrics, and to explore conceptual linkages between social prescribing and system/community outcomes.