Objectives: The aim of this study was to develop a Mental Health Questionnaire for Adults (MHQ-A) and investigate its validity and reliability. Method: A 73-item questionnaire was developed or selected by seven psychiatrists and a clinical psychologist. The questionnaire was administered to 351 subjects in Seoul and Gyeonggi area. The construct validity was investigated by performing principal component analysis, and the reliability was assessed in terms of the internal consistency using Cronbach’s alpha. Results: Principal component analysis revealed 15 factors underlying the psychiatric disorder and symptom category. Taken together, these 15 factors accounted for 64.023% of the common variance. Cronbach’s alpha was high, indicating that the reliability of the questionnaire was satisfactory. Every category showed a positive correlation with most psychological measures. Conclusion: These results indicate that the MHQ-A is a reliable and valid scale for assessing psychiatric disorders. Although there were many limitations, the present study has significance as the basis of future research.