Background: Understanding the mechanisms by which behavioral interventions exert their effects is important. Purpose: To examine behavioral mediators of weight loss in a sample of older adults participating in an evidence-based physical activity (PA) or nutrition intervention. Methods: Participants (n = 46) were randomized to a 12-week, group-based PA or nutrition intervention. Self-reported dietary behaviors and objectively measured PA and weight were measured at baseline and 12 weeks. A multiple mediator model examined whether PA, fruit and vegetable (F&V) consumption, and fat- and fiber-related behaviors mediated weight loss. Results: Improvements in fiber-related behaviors was a significant mediator of weight loss among those in the nutrition intervention group. PA, F&V consumption, and fat-related behaviors were not significant mediators. Discussion: Understanding how interventions exert their effects on health-related outcomes such as weight loss is critical in order to create the most effective interventions aimed at improving the health of older adult populations. Translation to Health Education Practice: Evidence-based behavioral interventions that utilize a lifestyle approach to elicit behavior change provide a feasible, low-cost option for practitioners that can easily be adapted and disseminated across various populations. Interventions focused on weight loss in older adults may want to make concerted efforts aimed at improving fiber-related behaviors.