Background P-factor and mentalizing theory and research set perspectives for transdiagnostic psychiatric treatments. Aims To test the effects of a low-cost mentalization-based health education program (the Thoughtful program) in an unselected waiting list population, from a psychiatric outpatient clinic in North Norway. Methods Waiting list patients were randomized (1:1 allocation): 79 patients in the control group were offered standard individual assessment and treatment. Seventy-nine patients in the intervention group were offered a one-day (six-hour) group-based Thoughtful course, plus individual assessment, and treatment. Results During a 6-month follow-up period, the number of patients in active assessment and treatment was 66% higher in the control group than in the intervention group. Self-reported patient questionnaire scores on mentalizing, well-being and suicidal ideation scores showed no significant changes in the control group. In the intervention group, significant changes were reported: Mentalizing scores improved by 72% and well-being scores improved by 55%. Suicidal ideation scores were not significantly changed. No adverse effects were registered. Limitations The results from this study should be interpreted with caution because of a small population size and low questionnaire response rate. The follow up period was limited to 6 months. Transfer of program information from intervention group patients to control group patients could not be guaranteed. It may be considered a limitation that diagnostic pattern analysis was not included in the study. Conclusions A low-cost transdiagnostic mentalization-based educational program in an outpatient psychiatry clinic may improve patient mentalizing and wellbeing without adverse effects and reduce the use of hospital services.