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Research has shown that asylum seekers, refugees, and internally displaced persons, including stateless persons, are at increased risk of poor mental health. Yet, only a few studies explicitly focus on stateless persons. This article examines the association between legal status and mental health among Akha, Lahu and Tai-Yai people in Northern Thailand. A convergent parallel mixed-methods design was adopted and primary data were collected in June 2020 by face-to-face, semi-structured interviews with persons who do not hold a nationality (n = 108). A distinction was made between registered (nationalityless) and unregistered (stateless) persons since only the former have a legal status in Thailand. The correlates of mental health were analyzed using descriptive statistics and logistic regression. Qualitative thematic analysis was employed to gain deeper insights into the relationship between legal status, ethnicity and mental health. The quantitative results weakly suggest that stateless persons are more likely to have poorer mental health than nationalityless persons, who are recognized as habitually resident in Thailand and are recorded in the country's civil registry. The qualitative results, however, show that the legal status afforded to nationalityless persons is considered important as it gives rise to hope and increases livelihood opportunities. Yet, respondents also pointed out that it is not comparable to citizenship. The odds of having poorer mental health are significantly higher for Lahu and Tai-Yai respondents. The importance of ethnicity is confirmed by the qualitative results. These further indicate that citizenship problems and ethnicity are deeply interrelated. Stigma, discrimination, and lack of social support are key risk factors identified by respondents. Disaggregating data to better understand the heterogeneity of persons without nationality could play a key role in accelerating efforts to resolve protracted citizenship problems and close ethnic gaps.