There are millions of children and young adults without parental care who depend on social services and residential, foster, relative, and kin carers for their care, relationships, and development. However, the social circumstances shaping their lives vary across countries due to differences in welfare states and child protection systems. Empirical evidence remains limited, particularly on European post-Soviet welfare states and West African mixed welfare systems. This study explores social work practice in alternative care in Latvia and Nigeria, focusing on how young people experience support while in care and during their transition to adulthood. A systems theory approach informed the research, linking the child/youth to micro, meso, exo, macro, and chrono-system levels of care. The research questions are: (1) What is the role of the social worker and care professionals in supporting young people in care and transitioning to adulthood? (2) How does care experience affect young people’s transition to adulthood? The study applied a sequential explanatory mixed-methods approach (QUANT → qual). Quantitative survey data were collected from 414 care-experienced young people in Nigeria (314 met the inclusion criteria) and 14 in Latvia using a 13-domain analytical instrument. The qualitative phase included interviews with five government social workers in Nigeria and open-text testimonies from Latvian care leavers. Five patterns emerged across the countries. First, kinship care provided the lowest levels of professional support. Second, although child participation was included in legislation and procedures, it disappeared at the point of contact. Third, support was weakest as children exited statutory care at age 16, with no youth mentor support available. Fourth, family placements fostered the greatest sense of belonging regardless of country or welfare system. The study found that improving child-centred social work practice does not require new theoretical frameworks, but consistent implementation of social work fundamentals, including genuine engagement, attention to less visible children, stronger aftercare services, and cross-regime learning to reduce discontinuity for youth leaving care. The study does not make a direct linear comparison between Latvia and Nigeria. Rather, it adopts a cross-system design examining social work practice and young people’s experiences within two distinct social, cultural, and welfare contexts to identify context-specific challenges, practices, and forms of support during the transition to adulthood.