This study evaluated the short-term impact of YWCA Encore, a mixed-modality group exercise and information support program for breast cancer patients situated in the community setting. Australian post-surgical breast cancer patients (N = 162) were assigned to either the 8-week Encore intervention or a Waitlist control. Intervention impact was assessed in terms of quality of life, cancer-specific distress, and social support. Changes in familiarity with exercise, self-efficacy and social support were tested as mediators of the intervention impact on quality of life. Overall adherence and satisfaction with the program were high. Significant enhancements at follow-up for quality of life and social support were evident for intervention compared with control participants. Familiarity with exercise and self-efficacy satisfied the requirements for mediation of quality of life. These findings provide evidence for psychosocial benefits of YWCA Encore multi-component program and support the use of such community-based programs for breast cancer survivors. The role of exercise familiarity and self-efficacy as probable mediators of the multi-component intervention is a critical finding and highlights the need for future investigations into the underlying mediating processes of similar interventions for cancer patients.