Depression and social isolation affect one in seven people over 65 and there is increasing recognition that social isolation adversely affects long-term health. Research indicates that interventions, which promote active social contact, which encourage creativity, and which use mentoring, are more likely to positively affect health and well-being. The purpose of this study was to evaluate a complex intervention for addressing social isolation in older people, embodying these principles: The Upstream Healthy Living Centre. Mentors delivered a series of individually-tailored activities, with support tailing off over time. Two hundred and twenty-nine participants were offered the Geriatric Depression Scale, SF12 Health Quality of Life, and Medical Outcomes Social Support scale at baseline, then 6 months and 12 months post intervention. Semi-structured interviews were conducted with 26 participants, five carers and four referring health professionals to provide a deeper understanding of outcomes. Data were available for 172 (75%) participants at baseline, 72 (53% of those eligible) at 6 months and 51 (55%) at 12 months. Baseline scores indicated social isolation and high morbidity for mental and physical health. The intervention was successful in engaging this population (80% of referrals were engaged in some form of activity). At 6 months, there were significant improvements in SF12 mental component, and depression scores, but not in perceived physical health or social support. At 12 months, there were significant improvements in depression and social support and a marginally significant improvement in SF12 physical component (p = 0.06), but the SF12 mental component change was not maintained. The qualitative data showed that the intervention was well-received by participants. The data indicated a wide range of responses (both physical and emotional), including increased alertness, social activity, self-worth, optimism about life, and positive changes in health behaviour. Stronger, 'transformational' changes were reported by some participants. Individual tailoring seemed to be a key mediator of outcomes, as was overcoming barriers relating to transport and venues. Key processes underlying outcomes were the development of a positive group identity, and building of confidence/self-efficacy. The Upstream model provides a practical way of engaging socially isolated elderly people and generating social networks. The data suggest a range of psychosocial and physical health benefits. Although there are limitations in attributing causality in uncontrolled studies, the data seem to indicate a reversal of the expected downward trends in some aspects of participants' health, and suggest that this approach is worth further investigation.