While broad evidence exists that positive psychology interventions are effective in increasing well-being, less is known about the working mechanisms behind this process. We examine the impact of subjective changes in affectivity (i.e., elicitation of positive emotions) and cognitive processes (i.e., the gaining of insights) in three variants of a pleasure-based placebo-controlled online intervention (N = 509 adults). The variants were designed that they have (1) a cognitive focus, (2) an emotional focus, or (3) both cognitive and emotional foci. We assessed happiness and depressive symptoms before the intervention, immediately after the intervention, and at follow-ups after two weeks, one month, and three months, and collected subjective ratings on potential working mechanisms. Findings indicated that both variants with a cognitive focus increased happiness in comparison to the control condition, whereas only those interventions that fostered the experience of positive emotions reduced depressive symptoms. Positive emotions mediated the effects of the intervention on happiness and depressive symptoms, whereas insights only mediated the effects on happiness. The findings support the important role of positive emotions in positive interventions and provide new evidence for the relevance of cognitive changes in such interventions.