Introduction Emergent mental illness during adolescence affects daily functioning, causing disruption to daily activities, routines, and patterns. Multiple inter-related personal, social and environmental determinants influence the onset, nature and subsequent course of those difficulties. Research suggests a bi-directional relationship exists between mental health and activity choices. Activity-focused interventions such as occupational therapy may improve adolescent mental health related outcomes. In this study, we identify and select which activity-related determinants should be prioritised in the development of an occupation therapy-based intervention for adolescents with emerging mental health difficulties using expert consensus. Method A modified two-round Delphi survey method was conducted with occupational therapists and researchers to ascertain a consensus opinion on the prioritisation of specific activity-related determinants that influence 16-to 17-year-olds’ Results Eighty-nine determinants were identified and prioritised. Fourteen of these were personal activity-related determinants including ‘types of activity’ in which young people engage, the ‘balance of activities’ in which they engage, their ‘over and under consumptions of activities’, and their ‘underdeveloped occupation-based coping skills’. The expert panel prioritised ‘personal self-confidence’, ‘values’, and ‘perception of confidence’ in relation to the activities adolescents do. Conclusions This study generated a detailed picture of the activity-related determinants that are important in adolescence, and aligns with the adolescent model of occupational choice. Our findings have potential to inform activity-related intervention development and policy. Further research is needed, particularly to understand young people’s perspectives on these determinants and to investigate the determinants that would benefit from further empirical research.