Background: Subjective quality of life (SQOL) in people with borderline personality disorder (BPD) is a marker of disease burden; a crucial treatment outcome; an indicator of psychosocial functioning; and a measure of interventions’ effectiveness. Given the dearth of consolidated data, the current study examined psychopathological determinants of global and domain-specific SQOL in people with BPD. Methods: Hierarchical regression models were employed to examine in BPD patients (n = 150) the relationships of the number of BPD diagnostic criteria; the co-occurrence of other personality disorders (PDs); depression; state and trait anxiety; suicidality; self-harming; alcohol and substance use disorders with SQOL indices, namely physical health, psychological health, social relationships, environment, overall QOL and overall health. SQOL was estimated using the WHOQOL-BREF instrument. Results: Co-existing symptomatology such as depression, state and trait anxiety, and personality pathology, namely the co-occurrence of other PDs, exhibited significant associations with global and domain-specific SQOL, albeit depression was the strongest determinant of the most SQOL domains. In contrast, the number of BPD diagnostic criteria and central illness features such as suicidality, self-harming behaviour, and impulsivity manifested through alcohol and substance use did not exhibit significant associations with any SQOL dimension. Conclusions: Comprehensive assessment of depressive symptoms should be regularly implemented in BPD services to facilitate early detection and treatment, thereby ensuring patients’ SQOL. Accordingly, tackling anxiety and other PDs co-occurrence through appropriate interventions can facilitate more effectively SQOL improvement. Our findings can be explained by the hypothesis that co-existing psychopathology such as depression, anxiety and co-occurrence of other PDs in BPD patients represent illness severity indices rather than comorbid disorders, and might fully mediate the effect of BPD traits on SQOL. Future mediation analysis is required to elucidate this hypothesis.