Objectives: We assessed whether an intervention based on acceptance and commitment therapy (ACT) and mindfulness was successful in promoting positive mental health by enhancing psychological flexibility. Methods: Participants were 93 adults with mild to moderate psychological distress. They were randomly assigned to the group intervention (n = 49) or to a waiting-list control group (n = 44). Participants completed measures before and after the intervention as well as 3 months later at follow-up to assess mental health in terms of emotional, psychological, and social well-being (Mental Health Continuum–Short Form) as well as psychological flexibility (i.e., acceptance of present experiences and value-based behavior, Acceptance and Action Questionnaire-II). Results: Regression analyses showed that compared with the participants on the waiting list, participants in the ACT and mindfulness intervention had greater emotional and psychological well-being after the intervention and also greater psychological flexibility at follow-up. Mediational analyses showed that the enhancement of psychological flexibility during the intervention mediated the effects of the intervention on positive mental health. Conclusions: The intervention is effective in improving positive mental health by stimulating skills of acceptance and value-based action.