Research has shown that traveling alone, suffering adverse stressful experiences, and living separately from family are risk factors for mental health problems in youth migrants. Against this background, unaccompanied minor migrants (UMMs) are one of the most vulnerable migrant populations. However, further investigation is needed to identify the factors related to the migration process that can impact the health of this population in Spain. This chapter aims to identify the relevant social determinants of health among newly arrived UMMs, encompassing factors before, during, and after migration. This aim is accomplished using a qualitative methodology to identify the voices of UMMs. Specifically, 15 semi-structured interviews were conducted as part of the national action-research Migrasalud project (II IN 190517 EN 162 FA 01). The interviews were transcribed, translated (from Arabic to Spanish), and analyzed through content analysis. The results showed that UMMs perceived themselves as having good health before the migration process. They often had unrealistic expectations before migrating, experienced adverse experiences during the journey, and, after arrival, had to cope with post-migration risk stressors such as socio-labor difficulties and discrimination, which affected their well-being and mental health. The findings also underline some relevant health and social challenges, such as the need to assess mental health care, considering the specific stages of the migratory cycle in the UMM population from a person-centered approach, the promotion of cross-cultural mental healthcare models, and the development of policies and services to address the effects of discrimination in Spain.