Objective: The well-being of migrant youth is a major global public health concern. This developmental stage is fraught with many challenges, with migrant youth suffering additional challenges as a result of migration. One avenue to better understand the psychological mechanisms that underpin the well-being of migrant youth is examining how mindsets–or implicit theories about the malleability of human characteristics–affect well-being. The aim of the current study was to test a conceptual model in which cognition malleability belief on well-being would be mediated by emotion regulation styles in two samples of migrant youth using two different measures of well-being. Methods: In Study 1, mainland China migrant youth (N = 735, Mean age = 13.89, SD = 1.23) completed a survey measuring demographics and cognition malleability belief, emotion regulation style (cognitive reappraisal, expressive suppression), and well-being (holistic well-being). In Study 2, Hong Kong migrant youth (N = 285, Mean age = 15.09, SD = 2.75) completed the same measures; however, well-being was assessed by the Life Satisfaction Scale. As different measures of the dependent variable (well-being) were used, two separate models were specified. Computations were performed with SPSS 22 and with the PROCESS macro. Results: Both studies demonstrated support for the conceptual model. As predicted, cognition malleability belief was associated with cognitive reappraisal, expressive suppression, and well-being of migrant youth from mainland China and Hong Kong. Cognitive reappraisal was positively associated with well-being, while expressive suppression was not significantly associated with well-being. The association between cognition malleability belief and well-being was mediated by cognitive reappraisal. Conclusion: Current findings provide avenues for future longitudinal and experimental research to test the efficacy of these mechanisms in changing beliefs about cognition malleability to promote the well-being of migrant youth.