Aim: MyLifeTracker is a session-by-session mental health outcome measure for young people aged 12 to 25 years. The aim of this study was to determine clinically significant change indexes for this measure that would identify developmentally appropriate thresholds. The study also aimed to determine expected change trajectories to enable clinicians to compare a client's progress against average rates of change. Methods: Participants comprised young people aged 12 to 25 years from both a clinical and a community sample from Australia. The clinical sample was 63 840 young people that attended a headspace centre. The non-clinical group was an Australian representative community sample of 4034 young people. Results: Clinically significant change indexes were developed for MyLifeTracker specific for age and gender groups by comparing clinical and non-clinical samples. Males and young people aged 12 to 14 years needed to reach higher scores to achieve clinically significant change compared to females and other age groups, respectively. MyLifeTracker expected change trajectories followed a cubic pattern for those with lower baseline scores of 0 to 50, whereas those with baseline scores of 51 and above had varying patterns. For those with lower baseline scores, expected change trajectories showed that stronger change was evident early in treatment, which then tapered off before accelerating again later in treatment. Conclusions: The development of MyLifeTracker benchmarks allows the measure to be used for Feedback Informed Treatment by supporting treatment planning and decision-making. This information can help clinicians to identify clients who are not on track or deteriorating and identify when clients are improving.