The purpose of this study was to describe the effectiveness of a practical group dynamics-based physical activity (PA) intervention and determine the mechanisms of PA change. Participants (n =115) were randomly assigned to the three-month Move More intervention or Enhanced Standard Care control. Self-reported PA and proposed mediators were assessed at baseline, three and nine months. Participants from both conditions increased their PA during the initial three months, F(2,67) = 24.67, p < .01. A significant time by condition interaction, F(2,67) = 4.25, p < .05, demonstrated that participants in the group dynamics condition sustained or further increased PA at nine months while those in the Enhanced Standard Care did not. Change in self-efficacy, outcome expectation-satisfaction, PA goals, and institutional support, significantly predicted increases in PA (p < .05) at three months, but not at nine months. These findings demonstrate the potential of adopting efficacious group dynamics principles from intensive PA interventions into a practical and effective clinical model.