Abstract Background Evidence-based policymaking is needed so that health systems can address the gap in care for children and adolescents facing mental health challenges. We describe the development of an open-resource dataset providing a comprehensive assessment of the needs for child and adolescent mental health care in Greece. Methods This study is a part of the Child and Adolescent Mental Health Initiative (CAMHI), a program aiming to enhance mental health care capacity for children and adolescents across Greece. A comprehensive, mixed-method, community-based research was conducted in 2022/2023 to examine the current state, needs, barriers, and opportunities according to multiple viewpoints. Participants consisted of children, adolescents (including underrepresented minorities), caregivers, schoolteachers, and health professionals. We surveyed geographically distributed samples to assess mental health symptoms, mental health needs, literacy and stigma, service use and access, professional practices, training background, and training needs and preferences. Focus groups were conducted with informants to reach an in-depth understanding of those topics. Results We surveyed 1,756 caregivers, 1,201 children/adolescents, 404 schoolteachers, and 475 health professionals. Fourteen focus groups were conducted with the general and professional community. A repository with quantitative and qualitative findings informing multiple topics is now available for researchers, policymakers, and society [ https://osf.io/crz6h/ ]. Discussion This resource offers valuable data for assessing the needs and priorities for child and adolescent mental health care in Greece. It is now freely available to consult, and is expected to inform upcoming research and evidence-based professional training. This initiative may inspire similar ones in other countries, informing methodological strategies for researching mental health needs. Funding The Stavros Niarchos Foundation.