Mindfulness-based interventions (MBIs) can reduce teachers’ stress. The purpose of this mixed-method study, conducted within the context of a randomized-control trial of an MBI for teachers, was to examine four potential ways by which the MBI reduced teacher stress, including by (1) increasing their efficacy for regulating emotion on the job; (2) improving their ways of coping with stress at work; (3) increasing their efficacy for forgiving colleagues and students at work following conflict, as well as the tendency to do so; and (4) increasing teachers’ tendency to feel compassion for people generally, and for challenging students in particular. Public school teachers (n = 59) were randomized to an MBI or a waitlist control condition. They completed surveys at pre/post/follow-up and interviews at post-program designed to assess their coping with work stressors and their appraisals of their most challenging students. Survey data showed that efficacy beliefs and the tendency to forgive changed from pre/post for teachers in the MBI, and partially mediated reductions in stress from baseline to 4-month follow-up. Interview results showed a trend for teachers in the MBI to report more adaptive strategies for coping with job stress, and a tendency to evaluate challenging students in a more positive affective light. Implications for MBIs in teacher professional development are discussed.