Objectives This study was undertaken to explore correlations between depression, alcohol drinking, psychiatric symptoms, and suicidal ideation, and to identify factors related to suicidal ideation in the mentally ill. Methods A survey was conducted using a self-reported questionnaire. Data were collected from July to September 2020. The study participants were 212 individuals with various major mental disorders. Results Depression was found to be positively correlated with alcohol drinking, psychiatric symptoms, and suicidal ideation. Alcohol use was positively correlated with psychiatric symptoms and suicidal ideation, and psychiatric symptoms were positively correlated with suicidal ideation. Factors influencing suicidal ideation included suicidal attempts, psychiatric symptoms, depression, and awareness of the mental health crisis hotline. Multiple regression analysis showed these variables explained 61% of suicidal ideation. Conclusion The study shows that to prevent suicide among the mentally ill, focus should be placed on reducing depression, alcohol drinking, and psychiatric symptoms and improving awareness of the mental health crisis hotline.