Borderline personality disorder (BPD) is a serious mental health condition associated with severe symptoms of distress and poor quality of life (QoL). Research outside the field of BPD suggests that ego-resiliency is negatively associated with psychopathology and positively associated with a range of positive life outcomes. Thus, ego-resiliency may be a valuable construct for furthering our understanding and treatment of BPD. However, the mechanisms linking ego-resiliency to psychopathology and QoL in relation to BPD have not been examined and explored by research. This study has addressed this gap in the collective knowledge by evaluating whether within-person associations between daily reports of positive affect (PA) and negative affect (NA) mediated the relationship between ego-resiliency, BPD symptom severity, and QoL. For 21 consecutive days, 72 women diagnosed with BPD completed end-of-day electronic assessments regarding ego-resiliency, PA and NA, symptom severity, and QoL. Multilevel structural equation modelling established that PA and NA were parallel mediators linking ego-resiliency with BPD symptom severity and QoL. As hypothesized, the path to QoL was stronger through PA than through NA. The mediation paths through NA and PA to BPD symptom severity were both significant, but their strength did not differ. Our findings align with the assertions of theories on emotion, thus suggesting a two-factor approach to PA and NA. Future research can build on these findings by developing psychotherapeutic interventions designed not only to reduce symptom severity but also to enhance PA in individuals with BPD and determine whether an increase in PA is associated with improved QoL.