Background/Objectives: This cross-national study focuses on adolescents who provide care and support to family members or significant others. Current evidence regarding their mental health and solutions to strengthen it is limited and mostly available in a few countries. The aim of this study is to evaluate the results of a primary prevention intervention for improving the mental health and well-being of adolescent young carers (AYCs) aged 15–17 years in six European countries. The intervention was based on a psychoeducational program and tools adapted from the Discoverer, Noticer, Advisor, and Values (DNA-V) model. Methods: We designed a randomized controlled trial with 217 AYCs participating in the study, either in the intervention or control group. Quantitative and qualitative data were collected via questionnaires at baseline, post-intervention, and a 3-month follow up. Results: The results were mixed, as positive improvements in primary (i.e., psychological well-being and skills) and secondary (school/training/work functioning) outcomes were shown by the experimental group but, in most cases, they were not statistically significant. The qualitative data supported positive claims about the intervention and its appropriateness for AYCs. Conclusions: The study implementation during the peak of the COVID-19 pandemic forced the consortium to adapt the design and may have influenced the results. More long-term studies are needed to assess similar mental health programs with this hard-to-reach target group.