Stress is a known contributor to psychosocial pathological conditions and reduction of well-being. The literature has highlighted the role of emotional schemas and psychological flexibility as psychological processes underlying this relationship in the short term. This two-wave longitudinal study analyses the mediating role of psychological flexibility and emotional schemas in the relationship between stress responses to COVID-19-related events and mental health 6 months later. Two hundred and seventy-six individuals were included in this study, completing measures of stress responses, emotional schemas, psychological flexibility and positive and negative mental health. Negative evaluation of emotions was a significant mediator in the longitudinal relationship between stress responses and negative mental health. Valued action and openness to experience mediate the longitudinal relationship between stress responses and positive mental health. This study underscores the importance of the different dimensions of psychological processes, suggesting that over time, more maladaptive psychological processes play a significant role in elucidating the relationship between stress responses and negative mental health. Similarly, it suggests that more adaptive processes may help explain the relationship between stress responses and positive mental health. This contributes to a better understanding of the stability of these processes, which is important for determining which intervention targets should be prioritised.