Background: Mental health disorders in the child and adolescent population are a pressing public health concern. Despite the high prevalence of psychopathology in this vulnerable population, the transition from Child and Adolescent Mental Health Services (CAMHS) to Adult Mental Health Services (AMHS) has many obstacles such as deficiencies in planning, organisational readiness and policy gaps. All these factors contribute to an inadequate and suboptimal transition process. A suite of measures is required that would allow young people to be assessed in a structured and standardised way to determine the on-going need for care and to improve communication across clinicians at CAMHS and AMHS. This will have the potential to reduce the overall health economic burden and could also improve the quality of life for patients travelling across the transition boundary. The MILESTONE (Managing the Link and Strengthening Transition from Child to Adult Mental Health Care) project aims to address the significant socioeconomic and societal challenge related to the transition process. This protocol paper describes the development of two MILESTONE transition-related measures: The Transition Readiness and Appropriateness Measure (TRAM), designed to be a decision-making aide for clinicians, and the Transition Related Outcome Measure (TROM), for examining the outcome of transition. Methods: The TRAM and TROM have been developed and were validated following the US FDA Guidance for Patient-reported Outcome Measures which follows an incremental stepwise framework. The study gathers information from service users, parents, families and mental health care professionals who have experience working with young people undergoing the transition process from eight European countries. Discussion: There is an urgent need for comprehensive measures that can assess transition across the CAMHS/AMHS boundary. This study protocol describes the process of development of two new transition measures: the TRAM and TROM. The TRAM has the potential to nurture better transitions as the findings can be summarised and provided to clinicians as a clinician-decision making support tool for identifying cases who need to transition and the TROM can be used to examine the outcomes of the transition process. Trial registration: MILESTONE study registration: ISRCTN83240263 Registered 23-July-2015 - ClinicalTrials.gov NCT03013595 Registered 6 January 2017.