The literature to date has documented the presence of challenges and barriers in mental health systems and services for children and adolescents worldwide. However, studies addressing this reality often do so in a fragmented, residual, incomplete, or generalized way, therefore hindering a comprehensive understanding of this complex phenomenon. The aim of this qualitative systematic review is to analyze the barriers and challenges affecting global mental health care for children and adolescents. Searches were made in the Web of Science, Scopus, and PubMed databases between 2018 and 2022 using terms connected with mental health, childhood, adolescence, and health systems. The search resulted in the extraction of 9075 articles, of which 51 were considered eligible for inclusion and complied with quality indicators. A number of closely related structural, financial, attitudinal, and treatment barriers that limited the quality of life and well-being of children and adolescents with mental health needs were found. These barriers included inadequate public policies, operational deficiencies, insufficient insurance coverage, privatization of services, stigma, lack of mental health literacy, lack of training, overburdened care, dehumanization of care, and lack of community and integrated resources. The analysis of these barriers displays that this treatment gap reflects the historical injustice towards mental illness and the disregard for real needs in these crucial stages, perpetuating a systematic lack of protection for the mental health of children and adolescents. The complexity of the disorders and the absence of public resources have resulted in a hodgepodge of mental health services for children and adolescents that fails to provide the continuing specialist health care they need.