Adolescents with cardiac diagnoses face unique challenges that can cause psychosocial distress. This study compares a Mindfulness-Based Stress Reduction (MBSR) program to a video online support group for adolescents with cardiac diagnoses. MBSR is a structured psycho-educational program which includes yoga, meditation, cognitive restructuring, and group support. A published feasibility study by our group showed significant reduction in anxiety following this intervention. Participants were randomized to MBSR or video online support group, and completed measures of anxiety, depression, illness-related stress, and coping pre- and post-6-session interventions. Qualitative data were obtained from post-intervention interviews. A total of 46 teens participated (mean 14.8 years; 63% female). Participants had congenital heart disease and/or cardiac device (52%), or postural orthostatic tachycardia syndrome (48%). Illness-related stress significantly decreased in both groups. Greater use of coping skills predicted lower levels of depression in both groups post-study completion. Higher baseline anxiety/depression scores predicted improved anxiety/depression scores in both groups. Each group reported the benefits of social support. The MBSR group further expressed benefits of learning specific techniques, strategies, and skills that they applied in real-life situations to relieve distress. Both the MBSR intervention and video support group were effective in reducing distress in this sample. Qualitative data elucidated the added benefits of using MBSR techniques to manage stress and symptoms. The video group format is useful for teens that cannot meet in person but can benefit from group support. Psychosocial interventions with stress management techniques and/or group support can reduce distress in adolescents with cardiac diagnoses.