Background <p>Borderline Personality Disorder (BPD) is a severe mental condition characterized by instability in self-image, emotions, and interpersonal relationships. Within mentalization models of BPD, the interaction between genetic vulnerabilities and early attachment-related adversity is understood to hinder the development of mentalization, contributing to core BPD symptoms. This study investigated early life stress (ELS) and mentalization difficulties in a Korean BPD sample and tested whether mentalization difficulties mediated their associations with BPD diagnosis and symptom severity. </p> Methods <p>We conducted a cross-sectional study with 90 individuals with BPD and 47 healthy controls using self-report measures of ELS, mentalization difficulties, and BPD symptoms. Group differences were examined, and a mediation model was tested using the PROCESS Macro.</p> Results <p>The BPD group showed higher levels of ELS across all domains and higher levels of most mentalization difficulties, with the exception of hasty mentalizing. Emotional abuse and bullying were associated with BPD diagnosis. Psychic equivalence and lack of emotional expression were associated with diagnosis, while psychic equivalence and lack of emotional awareness were associated with symptom severity. Mentalization difficulties partially mediated the links between ELS and both BPD diagnosis and symptom severity.</p> Conclusions <p>Preventive efforts targeting emotional abuse and bullying, along with interventions to address mentalization difficulties, may help inform preventive strategies and improve treatment outcomes. Results support the need for mentalization-based interventions for individuals with BPD in Asian populations.</p>