Mindfulness training improves mental health and psychological functioning. Although several questionnaires have been developed to measure mindfulness, the Five Facet Mindfulness Questionnaire (FFMQ) is currently one of the most widely used scales. However, uncertainty remains about whether the effects of mindfulness training can be unambiguously attributed to change in self-reported mindfulness. The present study was designed to answer three major questions: First, relative to a wait-list group, does participation in mindfulness training lead to changes in self-reported mindfulness among a mixed sample of individuals presenting stress-related problems, illness, anxiety, and chronic pain? Second, are changes in mindfulness associated with changes in psychological distress? Third, do changes in mindfulness mediate the effects of mindfulness training on the decrease in psychological distress? We used the French translation of the FFMQ in a Belgian sample. Relative to a wait-list control, mindfulness training led to a change in self-reported mindfulness and psychological distress. Further, changes in mindfulness mediated the effects of mindfulness training on a decrease in psychological distress.