Staging models with transdiagnostic validity across mood, psychotic, and anxiety disorders could advance early intervention efforts as well as our understanding of the common underpinnings of such psychopathology. However, there are few well-supported operationalisations for such transdiagnostic models, particularly in community-based samples. We aimed to explore the inter-relationships among mood, psychotic, and anxiety symptom stages, and their common risk factors to develop data-informed transdiagnostic stages. We included participants from the Avon Longitudinal Study of Parents and Children (ALSPAC), a prospective ongoing birth cohort study. We developed operational thresholds for stages of depressive, hypomanic, anxiety, and psychotic symptoms based on the existing literature, refined further by expert consensus. We selected 1b level as the primary stage or outcome of interest. This represents moderate symptoms that are likely to be associated with the onset of the need for clinical mental health care. We used questionnaire and clinic data completed by young people ages 18 and 21 years. We used descriptive methods and network analyses to examine the overlap among Stage 1b psychopathology. We then examined the patterns of relationships between several risk factors and 1b stages using logistic regressions. Among 3269 young people with data available to determine all symptom stages, 64.3% were female and 96% Caucasian. Descriptive and network analyses indicated that 1b level depressive, anxiety, and psychotic symptom stages were inter-related while hypomania was not. Similarly, anxiety, depressive, and psychotic 1b stages were associated with the female sex, more emotional and behavioral difficulties in early adolescence, and life events in late adolescence. Hypomania was not related to any of these risk factors. Given their inter-relationships and similar risk factors, anxiety, psychotic and depressive, symptoms could be combined to form a transdiagnostic stage in this cohort. Such empirical transdiagnostic stages could help with prognostication and indicated prevention in youth mental health.