This study investigates the mental health consequences of rural–urban migrants in Indonesia. We use the migrant economic assimilation model applied to our individual longitudinal data specifically designed to observe migrants' performance. Compared with urban non-migrants, this study finds that migrants' mental health conditions are persistently lower. Moreover, their mental health gap has worsened over the years since migration. Along with this pattern, we also found that migrants' earnings are persistently higher than those of urban non-migrants, with a decline in social support. This study is expected to enrich the literature on the assimilation of internal labour migration in developing countries.