Objective: This randomized controlled trial aimed to assess the efficacy of a structured cognitive–behavioral group program, Growing Pro-Social (GPS), in reducing cognitive distortions and early maladaptive schemas over time in male prison inmates. Method: A total of 254 participants were recruited from nine Portuguese prisons and allocated to receive GPS (n = 121) or treatment as usual (n = 133). Participants were assessed with self-report measures on cognitive distortions and early maladaptive schemas at baseline, during intervention, at post-treatment and at 12 months’ follow-up. Assessors were blind to group allocation. Treatment effects were tested with latent growth curve models. Results: At baseline, no significant differences between conditions were found. Results from latent growth curve models showed that condition was a significant predictor of change observed in all outcome measures over time. When compared with the control group, the treatment group showed a significant increase on adaptive thinking, and a significant decrease of cognitive distortions and early maladaptive schemas over time. Results also showed that treatment effects were maintained over time (12 months after GPS completion). Additionally, participants who completed the program presented higher improvements on cognitive distortions and early maladaptive schemas over time than noncompleters. Conclusion: This study showed that a structured cognitive–behavioral group program can have positive effects on the cognitive functioning of male prison inmates, by reducing cognitive distortions and the prominence of early maladaptive schemas.