Background In a recent trial, a 6-session intervention (BMT-CARE) integrating medical information with cognitive-behavioral strategies improved quality of life (QOL), mood, coping skills, and self-efficacy for family/friend caregivers of hematopoietic stem cell transplantation (HCT) recipients. This study examined whether improvements in coping and self-efficacy mediated the intervention effects on QOL and mood. Methods From December 2017 to April 2019, 100 caregivers of HCT recipients were enrolled into a randomized clinical trial of BMT-CARE versus usual care. Caregivers completed self-report measures of QOL (CareGiver Oncology Quality of Life questionnaire), depression and anxiety symptoms (Hospital Anxiety and Depression Scale), coping skills (Measure of Current Status), and self-efficacy (Cancer Self-Efficacy Scale-Transplant) at enrollment (before HCT) and 60 days after HCT. Causal mediation regression models were used to examine whether changes in coping and self-efficacy mediated intervention effects on QOL as well as depression and anxiety symptoms. Results Improvements in 60-day QOL in patients assigned to BMT-CARE were partially mediated by improved coping and self-efficacy (indirect effect, 6.93; SE, 1.85; 95% CI, 3.71-11.05). Similarly, reductions in 60-day depression and anxiety symptoms were partially mediated by improved coping and self-efficacy (indirect effect for depression, -1.19; SE, 0.42; 95% CI, -2.23 to -0.53; indirect effect for anxiety, -1.46; SE, 0.55; 95% CI, -2.52 to -0.43). Combined improvements in coping and self-efficacy accounted for 67%, 80%, and 39% of the total intervention effects on QOL and depression and anxiety symptoms, respectively. Conclusions Coping and self-efficacy are essential components of a brief psychosocial intervention that improves QOL and mood for caregivers of HCT recipients during the acute recovery period. LAY SUMMARY A 6-session program (BMT-CARE) focused on providing medical information, caregiving skills, and self-care and coping strategies has been previously reported to improve the quality of life and mood of caregivers of hematopoietic stem cell transplantation recipients in comparison with caregivers who receive care as usual. Using statistical models, this study suggests that learning coping skills and improving self-efficacy are the most essential components of this program that likely lead to better quality of life and mood for caregivers.