Enhancing positive affect (PA) and reducing negative affect (NA) are targets of positive psychology interventions, and well-being and positive functioning are core elements of mental health. However, the underlying temporal dynamics of these elements are unknown. This study aimed at identifying how a 42-day daily positive psychology intervention (PPI) impacts affective longitudinal dynamics compared to a control condition. This study employed an experience sampling method (self-observation diary including the Measurement of Affectivity: Valence/Activation scale, MAVA) with two non-randomised groups-a control group (n = 43) and an intervention group (n = 43)-taken from a community sample. Compared to the control participants, the PPI participants' trajectories for activated and deactivated NA showed an important decrease over the course of the PPI. The deactivated PA trajectory increased non-linearly across the PPI, whereas the activated PA trajectory showed a linear increase. This study suggests that PPIs change daily affective life.