Previous studies have demonstrated that insecure attachment patterns and a trait disposition toward negative affect and impulsivity are both associated with borderline personality disorder (BPD) features. According to attachment theory, insecure attachment patterns impart greater risk for the maladaptive personality traits underlying BPD. Hence, insecure attachment might be indirectly related to BPD through its association with these traits. The current cross-sectional study used structural equation modeling to compare two competing models of the relationship between adult attachment patterns, trait negative affect and impulsivity, and BPD features in a large nonclinical sample of young adults: (M1) attachment anxiety and avoidance are positively related to trait negative affect and impulsivity, which in turn, are directly associated with BPD features; and (M2) trait negative affect and impulsivity are positively related to attachment anxiety and avoidance, which in turn, are directly associated with BPD features. Consistent with attachment theory, M1 provided a better fit to the data than M2. However, only attachment anxiety, and not attachment avoidance, was significantly associated with negative affect and impulsivity. The results favored a model in which the relationship between adult attachment anxiety and BPD features is fully mediated by trait negative affect and impulsivity.