Cluster analysis of maltreatment-related mental health symptoms manifested by adolescents in foster care suggest the absence of an underlying taxonomic structure. To test this further, we investigated alignment between mental health symptom profiles derived through cluster analysis and nominal diagnosis of Borderline Personality Disorder (BPD) and Complex Post-traumatic Stress Disorder (C-PTSD), among a sample of 230 adolescents in long-term foster care. Nominal DSM-V BPD and ICD-11 C-PTSD caseness was estimated from Child Behaviour Checklist and Assessment Checklist for Adolescents score algorithms, and alignment of case assignment with previously-derived symptom profiles was examined. Nineteen BPD and three C-PTSD nominal cases were identified. Low C-PTSD prevalence reflected low concordance between PTSD and ‘disturbances in self organization’ (DSO) case assignment. The BPD and C-PTSD cases were aligned to more complex and severe symptom profiles. While the complex and severe presentations identified in the present study included core symptoms and clinical signs of BPD, they were also characterised by clinical-level inattention/over-activity and conduct problems. The present findings provide some support for the validity of the BPD construct for describing complex and severe psychopathology manifested by adolescents in foster care, and no support for the C-PTSD construct. However, the symptom profiles point to high variability in combinations of multiple symptom types that does not conform to traditional definitions of a ‘diagnosable’ mental disorder. Further research is needed to determine if complex post-maltreatment symptomatology can be validly conceptualised as one or more complex disorders.