Developmental psychopathology models, like the mentalization theory, have facilitated the theoretical understanding of trajectories that contribute to the expression and experience of borderline personality (BP) features. Mentalization theory posits that BP vulnerabilities and interpersonal difficulties arise from insecure attachment experiences and deficits in mentalizing ability. This study explored the intersections of attachment vulnerabilities in close relationships, mentalizing domains, and their combined impact on BP features and interpersonal competence in emerging adults. The sample included 742 emerging adults (18–25 years) from colleges across 23 Indian states, who completed an online survey measuring relationship-specific attachment anxiety and avoidance (mother, father, best friend), mentalization (self-related, other-related, motivation to mentalize), BP features, and interpersonal competence. Path analysis was conducted to examine (a) the association between relationship-specific attachment dimensions and BP features/interpersonal competence and (b) mentalization as a mediator. Attachment anxiety and avoidance in specific relationships were found to have direct positive effects on BP features and direct negative effects on interpersonal competence. However, the strength and significance of these effects were not similar across relationships, shedding light on unique contributions of relationship-specific attachment difficulties on BP features and interpersonal competence. Mentalization mediated these associations, with self-related mentalization uniquely affecting BP features and other-related mentalization influencing interpersonal competence. The findings underscore the importance of relationship-specific attachment and mentalization domains in BP vulnerabilities among emerging adults. They provide empirical support for developing targeted mentalization-based interventions to enhance self and interpersonal functioning.