Mental ill-health is now the leading cause of sickness absence and occupational incapacity in high-income countries. This study evaluated HeadCoach online manager training, designed to improve confidence, and managerial behaviors that create mentally healthy workplaces.A cluster randomized controlled trial was conducted comparing managers who received HeadCoach (N = 87) to waitlist control (N = 123). Managers' confidence and behavior were investigated at baseline, postintervention, and follow-up. Psychological distress of direct reports was evaluated.Confidence significantly increased postintervention only; however, per-protocol analyses indicated a significant improvement for program completers compared with control at both time points. Responsive and preventive behaviors significantly improved. Psychological distress of direct reports remained unchanged.HeadCoach online mental health training is an effective and scalable way to improve managers' confidence and workplace practices around mental health. The impact on direct reports remains unknown.