Teacher wellbeing and performance is affected by their ability to cope with the demands of the profession. This pilot non-randomized, waitlist-controlled study investigated the impact of a mindfulness intervention (Mindfulness-Based Stress Reduction) on teachers' wellbeing, self-regulation ability and classroom performance applying a mixed-method design. The sample was comprised of 32 German school teachers (93% female) which were distributed to a control and intervention group. Compared to the control condition, the intervention showed medium to high effect sizes on most outcome variables at post-test and results were sustained at follow-up. Mediation analyses showed that changes in mindfulness at post-test mediated changes in outcome variables at follow-up. Unexpectedly, the intervention seemed to negatively affect teacher engagement. Qualitative interviews highlighted the way mindfulness may influence teacher engagement and improve performance. Limitations of this study and future directions of research are discussed.