Leaders occupy the most critical roles in organizations. A growing body of research on mindfulness in the field of leadership suggests that mindfulness is a highly beneficial factor for various leader and organizational outcomes. The purpose of this study is to quantitatively review the research related to leader mindfulness from the perspectives of both self-report mindfulness levels and mindfulness interventions. Meta-analytic results from 54 independent samples and 9,414 leaders suggest that leader mindfulness significantly relates to leaders’ well-being (e.g., stress), relationships (e.g., relationship qualities, leadership styles), and job performance, as well as followers’ well-being and job performance. Moreover, we found strong support for the effectiveness of mindfulness interventions in improving leader well-being. We also found a significant difference between leader mindfulness levels before and after the intervention. The difference of post-test scores between the intervention and control groups was not statistically significant, though the effect was in the predicted direction. Through this quantitative review, we synthesize findings on leader mindfulness and mindfulness interventions for leaders, identify research gaps in the literature, and lay a solid foundation for advancing research on leader mindfulness.