Physical activity is critical in healthy weight loss, yet there is still much to be learned about psychosocial mechanisms of physical activity behavior change in weight loss. A sequential mixed methods approach was used to assess the physical and psychosocial impact of a 12-week cognitive-behavioral weight management program and explore factors associated with weight loss. Quantitative data (N = 55) revealed program completion rate and mean percentage weight loss that compare favorably with other interventions, and differential psychosocial impacts on those losing more weight. Telephone interviews (n = 11) revealed four potential mechanisms for these differential impacts: (a) fostering accountability, (b) balancing perceived effort and success, (c) redefining "success," and (d) developing cognitive flexibility. (Contains 4 tables.)