Objective While there is growing interest in improving patient activation in general medical health services, there are too few randomized controlled trials in mental health settings which show how improvement can be achieved. Using the Patient Activation Measure-13 (PAM-13), we aimed to assess the effect of pre-treatment, peer co-led educational intervention on patient activation. Secondary outcomes included measures of patient satisfaction, well-being, mental health symptoms, motivation, and treatment participation. Methods Patients from two community mental health centres were randomized to a control group (CG, n = 26) receiving treatment as usual, or an intervention group (IG, n = 26) consisting of a four-hour group educational seminar (aiming to encourage patients to adopt an active role in their treatment) followed by treatment as usual. Results Only the IG improved on PAM-13, at one- and four-month follow-ups. The intervention had significant effects on patient satisfaction and treatment participation, compared to CG. Conclusion Providing pre-treatment, peer co-led education improves patient activation in community mental health care settings. Practice implications The use of peers as co-educators may contribute to a different mental health care delivery, ensuring patient activation and participation in treatment. Further studies should examine peers’ needs for supervision, challenges for the services, long-term and cost-benefit effects.