Objective: The Make Better Choices 1 trial demonstrated that participants with unhealthy diet and activity behaviors who were randomized to increase fruits/vegetables and decrease sedentary leisure achieved greater diet and activity improvement than those randomized to change other pairs of eating and activity behaviors. Participants randomized to decrease saturated fat and increase physical activity achieved the least diet-activity improvement. This study examined which psychological mechanisms mediated the effects of the study treatments on healthy behavior change. Methods: Participants (n = 204) were randomized to 1 of 4 treatments: increase fruits/vegetables and physical activity; decrease saturated fat and sedentary leisure; decrease saturated fat and increase physical activity; increase fruits/vegetables and decrease sedentary leisure. Treatments provided 3 weeks of remote coaching supported by mobile decision support technology and financial incentives. Mediational analyses were performed to examine whether changes in positive and negative affect, and self-efficacy, stages of readiness to change, liking, craving and attentional bias for fruit/vegetable intake, saturated fat intake, physical activity, and sedentary leisure explained the impact of the treatments on diet-activity improvement. Results: Greater diet-activity improvement in those randomized to increase fruits/vegetables and decrease sedentary leisure was mediated by increased self-efficacy (indirect effect estimate = 0.04; 95% bias corrected CI, 0.003–0.11). All treatments improved craving, stage of change and positive affect. Conclusion: Accomplishing healthy lifestyle changes for 3 weeks improves positive affect, increases cravings for healthy foods and activities, and enhances readiness to make healthy behavior changes. Maximal diet and activity improvement occurs when interventions enhance self-efficacy to make multiple healthy behavior changes.