Stress resilience is the maintenance of mental health despite adversity. We have predicted that a tendency to appraise stressors in a realistic to slightly unrealistically positive fashion (positive appraisal style, PAS) is prospectively associated with more resilient outcomes; that PAS is a proximal and integrative resilience factor, mediating the pro-resilience effects of other protective factors (e.g., social support); and that PAS is modifiable, with changes in PAS leading to corresponding changes in resilience. In two independent observational samples (N=132 and N=1034), we find PAS to predict resilience over three and more years and to mediate the positive effects of social support. Analyzing the effects of a multi-component intervention (N=232) that targets a broad set of resilience factors, we find that the intervention increases PAS and that this prospectively mediates the intervention-induced increases in resilience. This establishes PAS as a proximal and plastic resilience factor with likely causal effects on resilience.