Increased time spent in nature can enhance physical health and mental wellbeing. The UK Government's '25 Year Environment Plan' recommends extending the health benefits of contact with nature to a wider group of people, including those with mental health challenges. This study investigated whether nature-based interventions (NBIs) for people with mild mental health challenges could generate a positive social return on investment (SROI). Between May 2017 and January 2019, 120 participants at six outdoor sites in Wales participated in a 6 to 12-week NBI, which consisted of a weekly 2- to 4-h session. Quantitative data were collected from baseline and follow-up questionnaires identifying participant demographics and measuring mental wellbeing, physical activity, self-efficacy, and social trust. Wellbeing valuation generated a range of social value ratios by applying the Housing Association Charitable Trust (HACT) Social Value Calculator (SVC 4.0) and HACT Mental Health Social Value Calculator (MHSVC 1.0). Seventy-four participants (62%) completed follow-up questionnaires at 6 months. SROI ratios were calculated using the SVC for physical activity, self-efficacy, and social trust. The MHSVC generated social value ratios for mental wellbeing. The base case results revealed a positive social value ratio for participants, ranging from British Pound Sterling (GBP) 2.57 to GBP 4.67 for every GBP 1 invested, indicating favourable outcomes in terms of value generated.