Objective: Subjective well-being is a crucial variable for mental health practitioners. This study examines the influence of therapists' attachment dimensions and self-reported reflective functioning on their perceived well-being. Further, it examines if reflective functioning mediates the association between attachment insecurity and well-being. Method: A total of 416 experienced psychotherapists were enrolled in this cross-sectional study, and completed self-report measures of attachment insecurity, reflective functioning, and well-being. We tested the hypothesized mediation model with path analysis that examined indirect effects. Results: Both attachment anxiety and avoidance dimensions had a significant negative association with perceived well-being with small to medium effects. "Certainty" in reflective functioning had a small positive effect on therapist well-being. Reflective functioning mediated the association between insecure attachment dimensions and well-being, suggesting that therapist's lower ability to mentalize may partially account for the effects of higher attachment insecurity on lower well-being. Conclusion: The well-being of psychotherapists with greater insecure attachment may deserve special attention, and therapists' mentalizing capacities may be targeted by researchers and trainers as a core ability to be cultivated in order to preserve therapists' professional and personal resources.