Background Mindfulness‐based cancer recovery (MBCR) and supportive expressive group therapy (SET) are two well‐validated psychosocial interventions, but they have not been directly compared, and little is known about long‐term outcomes. This comparative effectiveness study measured the effects of these two interventions immediately following the groups and for 1 year thereafter in distressed breast cancer survivors. Methods Two hundred fifty‐two distressed Stage I–III breast cancer survivors were randomized into either MBCR or SET. Women completed questionnaires addressing mood, stress symptoms, quality of life, social support, spirituality and post‐traumatic growth before and after the interventions, and 6 and 12 months later. Results Immediately following the intervention, women in MBCR reported greater reduction in mood disturbance (primarily fatigue, anxiety and confusion) and stress symptoms including tension, sympathetic arousal and cognitive symptoms than those in SET. They also reported increased emotional and functional quality of life, emotional, affective and positive social support, spirituality (feelings of peace and meaning in life) and post‐traumatic growth (appreciation for life and ability to see new possibilities) relative to those in SET, who also improved to a lesser degree on many outcomes. Effect sizes of the time × group interactions were small to medium, and most benefits were maintained over 12 months of follow‐up. Conclusions This study is the first and largest to demonstrate sustained benefits of MBCR in distressed breast cancer survivors relative to an active control. MBCR was superior to SET for improving psychological well‐being with lasting benefits over 1 year, suggesting these women gained long‐lasting and efficacious tools to cope with cancer. Trial Registration Registered on clinicaltrials.gov number NCT00390169, October 2006. Copyright © 2016 John Wiley & Sons, Ltd.