The increased popularity of mindfulness-based interventions and the growing body of empirical evidence confirming the positive effects of these interventions on well-being warrant more research to determine if the effects are indeed related to learning mindfulness. The present study extends previous studies, by examining whether and how changes in five core aspects of mindfulness are related to changes in the report of negative and positive affect during an 8-week course of mindfulness-based cognitive therapy. The study was performed in 64 individuals from the community with mild to moderate psychological problems. Data were collected by self-report questionnaires before and directly after the training. Results showed significant decreases in negative affect and increases in positive affect. We also found significant increases in four of the five aspects of mindfulness. Importantly, changes in mindfulness were significantly associated with improved affect, with a distinct pattern found for positive and negative affect. Hereby, our findings extend previous research by showing that learning distinct aspects of mindfulness is differently related to an improved positive affect and a decreased negative affect. Future randomized controlled trials with a larger sample and longer follow-up period are needed to replicate these findings.