Mental health is more than the absence of mental illness. Rather, both well-being (positive mental health) and mental illness are actually two related continua, with higher levels of well-being defined as “flourishing.” This two-continua model and existing studies about the impact of flourishing on psychopathology underscore the need for interventions that enhance flourishing and well-being. Acceptance and Commitment Therapy (ACT) is a model of cognitive behavioral therapy that aims not only to reduce psychopathology but also to promote flourishing as well. This is the first study to evaluate the impact of ACT on flourishing. A post-analysis was conducted on an earlier randomized controlled trial of a sample of adults with depressive symptomatology who participated in a guided self-help ACT intervention. This post-analysis showed a 5%–28% increase of flourishing by the participants. In addition, the effects on flourishing were maintained at the three-month follow-up. When compared to participants in a control group, the flourishing of the ACT-trained participants increased from 5% to about 14% after nine weeks. In addition to levels of positive mental health at baseline, an increase of psychological flexibility during the intervention was a significant predictor of flourishing at the three-month follow-up.