A major barrier to improving care effectiveness for mental health is a lack of consensus on outcomes measurement. The International Consortium for Health Outcomes Measurement (ICHOM) has already developed a consensus-based standard set of outcomes for anxiety and depression in adults (including the Patient Health Questionnaire-9, the Generalised Anxiety Disorder 7-item Scale, and the WHO Disability Schedule). This Position Paper reports on recommendations specifically for anxiety, depression, obsessive-compulsive disorder, and post-traumatic stress disorder in children and young people aged between 6 and 24 years. An international ICHOM working group of 27 clinical, research, and lived experience experts formed a consensus through teleconferences, an exercise using an adapted Delphi technique (a method for reaching group consensus), and iterative anonymous voting, supported by sequential research inputs. A systematic scoping review identified 70 possible outcomes and 107 relevant measurement instruments. Measures were appraised for their feasibility in routine practice (ie, brevity, free availability, validation in children and young people, and language translation) and psychometric performance (ie, validity, reliability, and sensitivity to change). The final standard set recommends tracking symptoms, suicidal thoughts and behaviour, and functioning as a minimum through seven primarily patient-reported outcome measures: the Revised Children's Anxiety and Depression Scale, the Obsessive Compulsive Inventory for Children, the Children's Revised Impact of Events Scale, the Columbia Suicide Severity Rating Scale, the KIDSCREEN-10, the Children's Global Assessment Scale, and the Child Anxiety Life Interference Scale. The set's recommendations were validated through a feedback survey involving 487 participants across 45 countries. The set should be used alongside the anxiety and depression standard set for adults with clinicians selecting age-appropriate measures.