Objectives: There is a growing interest in mental well-being as a vital outcome in clinical practice in addition to mental illness. The model of sustainable mental health (SMH) was recently introduced to delineate how interventions can improve mental health by targeting barriers and resources of adaptation to life stressors, improving the ability to adapt and thereby reducing mental illness and improving mental well-being. The aim of the current study is to empirically validate the conceptual model of SMH as well as the assumed indirect role of ability to adapt. Methods: This study used an existing dataset of the general population with self-reported reduced well-being due to the corona crisis (n = 849, mean age 53 years, SD = 15). Measurements of mental illness (depression and anxiety), mental well-being, ability to adapt, a specific barrier for adaptation (i.e., repetitive negative thinking), and a specific resource for adaptation (i.e., positive reframing) were included. Structural equation modeling was used to assess both the structural validity of the model and the indirect effect of ability to adapt. Results: An acceptable to good fit was found for the model of SMH and all paths between the proposed elements of the model were significant and in the hypothesized direction. Ability to adapt served as an indirect pathway trough which repetitive negative thinking (B = 0.149, 95% confidence interval [CI] = 0.016–0.028) and positive reframing (B = 0.163, 95% CI = 0.065–0.123) were linked with mental illness and mental well-being. Conclusion: The current study provides the first empirical support of the internal validity of the model of SMH in a sample of the general population with reduced well-being, suggesting that barriers and resources to adaptation have an effect on mental illness and mental well-being through the ability to adapt. The model of SMH may therefore be a good model to use in research and clinical practice for developing, implementing, and evaluating a balanced treatment approach targeting both barriers and resources for adaptation.