Purpose: The ‘My youth mental health Session Experience’ (MySE) measure was developed by headspace, Australia’s National Youth Mental Health Foundation, in collaboration with young people, for use as a routine session experience measure across its national centre service network. The measure fills a gap in measures needed to implement measurement-informed care in youth mental health care. Participants and Methods: Routinely collected data from 37,201 young people aged 12 to 25 years who commenced an episode of care at one of the 150 headspace centres between 1 July 2021 and 30 June 2022 were used to validate the five-item measure. Results: MySE demonstrated high internal consistency invariant over age and gender groups. There was one latent factor of session experience that all MySE items relate to, although this factor does not adequately capture all the information present in the individual items. A significant age effect showed that young adults reported more positive session experiences than adolescents. Conclusion: MySE demonstrated strong psychometric properties and is suitable for use in youth mental health care as a routine session-by-session experience measure. Such measures are needed to routinely inform clinicians of how young people are experiencing their treatment sessions, thereby contributing to better retention, engagement, and client outcomes through measurement-informed care.