The efficacy of several multicomponent positive psychology interventions (PPIs) have been demonstrated, but little is known about its possible mechanisms of change. We examined (1) the efficacy of an email guided self-help PPI on six core well-being processes (positive emotion, use of strengths, optimism, self-compassion, resilience and positive relations) and (2) the mediating role of these processes on mental well-being, anxiety and depressive symptoms. Adults ≥ 18 years were recruited in the general population and randomized into the intervention (n = 137) or wait-list control group (n = 138). Repeated measures analyses showed that the intervention group improved significantly more than the control group on all six processes. Improvement (t0-t1) on each process statistically mediated improvement (t0-t2) on mental well-being, anxiety and depressive symptoms. Simple-mediation analyses revealed small to moderate effect sizes. Multiple-mediation analyses revealed most pronounced results for positive relations and self-compassion, suggesting that these processes might be key mechanisms in promoting well-being.