Parent–child informant discrepancies on psychopathology provide important knowledge on the parent–child relationship and the child’s mental health, but mechanisms underlying parent–child informant discrepancies are largely unknown. Therefore, we investigated the relationship between attachment problems and mentalizing capacity and parent–child informant discrepancies on borderline personality disorder (BPD) severity, internalizing, and externalizing pathology in a clinical sample of 91 adolescent girls with BPD and their parents. Results showed that more attachment problems to parents and peers were related to adolescents reporting more severe BPD than parents. Adolescents who described more internalizing symptoms relative to parents, reported more parental attachment problems, but enhanced peer attachment, suggesting those adolescents who do not feel recognized by their parents might turn to their friends. When parents rated adolescents higher on externalizing behaviors, the adolescent reported more attachment problems to parents and lower mentalizing capacity, indicating that this sub-group of adolescents may reflect less about how their behavior affects others.