Although mindfulness plays an important role in the treatment of borderline personality disorder (BPD), few studies have investigated whether individuals with BPD show difficulties in mindfulness. Those studies have relied exclusively on questionnaire data. Here, we used a more objective, behavioral assessment of mindfulness difficulties in BPD that allowed us to distinguish difficulties in mindfulness, prolonged mind-wandering, and difficulties in awareness of mind-wandering. Twenty-five individuals with BPD and 28 healthy controls performed two mindful breathing tasks. Frequency of mindfulness versus mind-wandering was assessed in the experimenter-prompted mindfulness task, where participants were interrupted at random intervals and asked whether they had just been mindful or mind-wandering. Awareness of mind-wandering was assessed in the self-prompted mindfulness task, where participants were asked to press a computer key whenever they noticed mind-wandering. Length of mind-wandering was assessed in both tasks. In individuals with BPD, we found a lower frequency of mindful episodes. In addition, individuals with BPD indicated longer mind-wandering episodes in the experimenter-prompted task. However, individuals with BPD detected mind-wandering episodes as accurately as healthy controls in the self-prompted task. Finally, difficulties in mindfulness observed in the behavioral tasks positively correlated with participants’ reports of mindfulness difficulties in everyday life. Using behavioral tasks to specify difficulties in mindfulness in BPD, this study indicated that although individuals with BPD mind-wander longer and more often, their awareness of mind-wandering appear unaffected. This indicates that mindfulness difficulties can be reported by individuals with BPD and that therapists can build on this awareness through mindfulness-based treatments of BPD.