Measurement-based care (MBC), the use of routine assessment to guide clinical decision-making, has the potential to significantly enhance the quality of mental health services for youth by improving the detection and prevention of harm. Concerns exist, however, that widespread efforts to implement MBC may have a negative impact on youth mental health care. We explore both perspectives by describing how MBC can be leveraged as a tool to detect and prevent harmful treatment in youth and how misapplication of MBC also has the potential to cause harm. This discussion focuses on how MBC can help therapists identify harm caused by ineffective practices (i.e., deterioration or no symptoms changes), poor client engagement (e.g., early drop-out, poor alliance), and adverse events (e.g., injury caused by treatment). We also discuss multi-level applications of aggregated MBC data to prevent harm through workforce development and organizational and health systems quality and cost of care improvement initiatives. Misapplication of MBC by failing to adhere to MBC best practices, overreliance on data above clinical judgement, providing insufficient organizational support, and using MBC data to justify cost containment are all discussed as possible sources of harm. We conclude with a discussion of future directions for research needed to advance the application of MBC to the prevention of harm in youth psychotherapy.