The present study aimed to investigate the affect-cognition interplay in young and older adults by studying prospective memory (PM), the realisation of delayed intentions. While most previous studies on the topic were conducted in the laboratory, we examined the influence of naturally occurring affect on PM tasks carried out in participants' everyday lives. For seven consecutive days, participants were asked to rate their affective state nine times per day and send text messages either at specific times (time-based PM) or when a particular event occurred (event-based PM). Results showed that within-participants changes in valence from more positive to more negative affect were associated with decreased PM performance. This was similarly true for young and older adults. The design used allowed linkage of within-participants fluctuations of affect and cognitive functions, constituting a methodological advancement. Results suggest that positive affect has the potential to improve cognitive functioning in everyday life.