The purpose of this study was to develop and test the psychometric properties of the Mental Health Assessment Tool for Older Thai Adults (MHAT-T), a 32-item self-report questionnaire. Development of the MHAT-T was based on an exploratory sequential mixed methods design, where the results of the first method (qualitative) were used to develop the mental health assessment tool for older Thai adults. The initial questionnaire was composed of 100 items and carried out among 1,266 older Thai adults (aged 60 years and over), including Buddhists, Muslims, and Catholics in the northeastern, northern, central, and southern Thailand, and the Bangkok Metropolitan Area. The exploratory factor analysis, with an acceptable factor structure model, yielded 32 items aligned with five factors: family attachment, mental quality, community dignity and support, mental status, mental capacity. The preliminary psychometric properties demonstrated the internal consistency of the MHAT-T was at a high range of 0.76 to 0.87 and acceptable for a new instrument. The MHAT-T had an excellent content validity index score of 0.99. Test-retest reliability at two weeks was fair (0.54 to 1.00, p>0.001). A score of 92 of 106 points was classified as normal mental health. The MHAT-T served adequately as a newly captured construct and should be offered as a tool for assessing mental health among older Thai adults in the community.