The article describes an investigation of the nature, patterns and complexity of carer-reported mental health symptoms for a population sample (N = 230) of adolescents (age 12–17) placed in long-term foster and kinship care following chronic and severe maltreatment. Two cluster analyses of Child Behaviour Checklist DSM-oriented (CBCL-DSM) and Assessment Checklist for Adolescents sub-scale scores of clinical cases were performed. The first yielded 8 profiles of attachment- and trauma-related symptoms as measured across eight ACA scales (N = 113 cases). The second yielded 11 profiles of a broader range of symptoms, as measured across five CBCL–DSM and five ACA sub-scales (N = 141 cases). The symptom profiles derived from both cluster analyses are differentiated more by symptom severity and complexity, than by symptom specificity – suggesting that trauma- and attachment-related symptomatology does not conform to a taxonomy of discrete disorders. Five of the 11 CBCL-DSM/ACA profiles describe severe and complex symptomatology that does not correspond to discrete DSM-5 or ICD-11 diagnoses. Accurate measurement and formulation of clinical phenomena is an essential component of evidence-based psychological and psychiatric practice. Clinicians who carry out mental health assessments of children and adolescents in care should be aware of the limits of the diagnostic classification systems for formulating complex attachment- and trauma-related symptomatology.