Background The percentage of people in Wales experiencing severe mental health issues more than doubled during the COVID-19 pandemic. Additionally, hundreds of people in Wales wait more than a year for help with their mental health. The EmotionMind Dynamic (EMD) programme is a six-session programme over 3 months involving self-reflective introspection, self-analysis, problem solving, goal setting, and action taking. Furthermore, this programme challenges negative self-perception and increases self-awareness, self-confidence, and self-esteem. We aimed to estimate the social return on investment of EMD lifestyle coaching, both face-to-face and online formats, by comparing the costs of running the programme with the social value generated from clients as measured by improvement in self-confidence and mental wellbeing. Methods We included 15 clients from previous face-to-face EMD coaching and 17 clients from a new online version of EMD. For face-to-face clients, quantitative data were collected retrospectively with a one-time only questionnaire. For new online clients, quantitative data were collected from baseline and follow-up questionnaires. Qualitative data were collected after intervention from interviews with both groups. Outcomes from questionnaires for both groups included changes in mental wellbeing measured with the Short Warwick Edinburgh Mental Wellbeing Scale (SWEMWBS) and self-efficacy assessed with the General Self-Efficacy Scale (GSES). Findings For every £1 invested, lifestyle coaching generated social values ranging from £4·12 to £7·08 for face-to-face clients compared with £2·37 to £3·35 for online participants. Quantitative and qualitative data from questionnaires and interviews indicated that many clients had improved mental wellbeing and self-efficacy. All 15 face-to-face clients and 11 (65%) of 17 online clients reported an increase of 5 points or more on the SWEMWBS questionnaire. Similarly, all 15 face-to-face clients and ten (59%) of 17 online clients reported an increase of 5 points or more on the GSES questionnaire. Interpretation The results showed that both face-to-face and online formats of the EMD lifestyle coaching generated a positive social return on investment ratios. With continued long waiting lists for people with mental health challenges, face-to-face and online lifestyle coaching might become more essential across statutory, private, and third sectors to meet the growing demand for mental health support. Funding Accelerate: the Welsh Health Innovation and Technology Accelerator.