Physical distancing practices during the COVID-19 global pandemic contributed to a high degree of social isolation among older adults. To reduce loneliness and other ill effects of social isolation, public health experts recommended outdoor social gathering, with physical distancing. Adopting a case study approach, we explored how social aspects of group outdoor health walks (GOHWs) mitigate social isolation for older adults and improve individual social wellbeing. We used semi-structured interviews to understand the experiences of social isolation and social relationships in nine older (50–80 s) adults participating in a GOHW in Scotland, United Kingdom (UK). Verbatim transcripts were analysed through an iterative process of thematic analysis carried out by an interdisciplinary team of qualitative researchers from environmental psychology, medicine, and geography. Themes provide insight into the social dimensions of GOHWs, the mediating effects of social experiences, and the contribution these make to individual social wellbeing. GOHWs provide opportunities to be part of a group and attend to the needs of inexperienced or physically challenged individuals. Being part of the group walk fosters casual interpersonal interactions through spontaneous mixing during and after the walk. This programmatic structure counters loneliness, engenders pleasurable anticipation of regular contact with others, supports physical activity, and fosters group cohesion. These in turn contribute to individual social wellbeing, including expanding social networks, meaningful relationships, a sense of belonging, and acting on empathy for others. GOWHs may be beneficial for mitigation of social isolation as we emerge from the COVID-19 pandemic. Findings were used to propose a conceptual model to parse social constructs and inform selection or development of quantitative social measures for future studies of nature-based interventions such as GOHWs.