Purpose: This study was conducted to develop a monitoring tool to manage unregistered mentally ill patients at community mental health centers (M-MUMI). Methods: The Delphi method was used in this study. The participants of this study were 27 psychiatric experts. In the first Delphi round, content analysis was conducted. In the second Delphi round, Kendall’s W rank and weight rank were used. The third Delphi round was analyzed based on the content validity ratio (CVR). Results: The finally developed M-MUMI consisted of 2 domains, 7 categories, and 22 items. Assessment of the present status consisted of 4 basic elements affecting daily life, 5 psychological statuses, 2 elements related to the utilization of social service, and 3 causes for the refusal of registration. The registration strategy of the individual cases consisted of 2 human resources to help registration, 2 additional social services for registration, and 4 individual needs to induce registration. Conclusion It is important to manage unregistered mentally ill patients at the community mental health centers. We hope that the developed M-MUMI will help to monitor unregistered mentally ill patients how to maintain mental health in the community.