There is growing evidence that fostering mental well-being and flourishing might effectively prevent mental disorders. In this study, we examined whether a 9-week comprehensive positive self-help intervention with email support (TL-E) was effective in enhancing well-being and flourishing and decreasing anxiety and depressive symptoms in a non-clinical sample.A total of 275 participants with low or moderate well-being (mean age=48years, 86% female) were randomly assigned to a TL-E (n=137) or wait-list control group (WL; n=138). Participants completed online self-reporting questionnaires at baseline and at 3, 6 and 12months.Repeated measure analyses revealed significant more improvement on mental well-being (F=42.00, p≤0.001, d=0.66, 95% CI=0.42–0.90), anxiety (F=21.65, p≤0.001, d=0.63, 95% CI=0.39–0.87) and depression (F=13.62, p≤0.001, d=0.43, 95% CI=0.19–0.67) in the TL-E group versus the WL group. The proportion of flourishing in the TL-E group increased from 7 to 30% after 3months (NNT=5.46) and to 34% after 6months (NNT=5.25). All within group effects were maintained up to 12months. We found no meaningful dose-response relationship for adherence, nor a clear moderator pattern.It is unknown whether results were influenced by the email support that accompanied the self-help intervention since TL-E was only compared to a wait-list condition. The generalizability of the findings is limited by the self-selected sample of mainly higher-educated women.A guided positive self-help intervention might be considered as a new mental health promotion strategy because it has the potential to improve well-being up to the status of flourishing mental health, and to decrease anxiety and depressive symptomatology.