There is growing interest in green social prescribing and connecting with nature-based activities to promote social cohesion along with improving levels of health, wealth and well-being. The Outdoor Partnership is a third sector organisation based in North Wales offering nature based social prescribing interventions. Individuals experiencing poor mental health and wellbeing are referred from GPs, community mental health services, and third sector organisations to the ‘Opening the Doors to the Outdoors’ (ODO) programme which is a 12-week outdoor walking and climbing green prescribing intervention. The purpose of the ODO programme is to provide a supportive environment to increase levels of physical activity among participants leading to improvements in overall health and mental wellbeing while promoting socialisation among peers. In this evaluation of a preventative green social prescribing intervention, a mixed method social return on investment (SROI) approach used quantitative and qualitative data from ODO participants. Data collection took place from April 2022–November 2022. Mental wellbeing data was collected at baseline and at 12 weeks using the Short Warwick Edinburgh Mental Wellbeing Scale, a social trust question, an overall health question, and the International Physical Activity Questionnaire- short form. Baseline and follow-up data was available for 52 ODO participants. Results indicate that for every £1 invested in the ODO programme, social values ranging from £4.90 to £5.36 were generated.