Attention-deficit/hyperactivity Disorder (ADHD) and borderline personality disorder (BPD) have overlapping symptoms. We proposed that excessive spontaneous mind wandering (MW-S) might reflect a component of psychopathology that distinguishes ADHD from BPD. Using a questionnaire measure of MW-S and an experience sampling method, we investigated MW-S in daily life, in 28 ADHD, 19 BPD, 22 comorbid ADHD+BPD, and 29 control females. The clinical groups reported heightened frequency and intensity of MW-S compared to controls, but no differences from each other. When controlling for depression and anxiety, significant differences only persisted between controls and ADHD, who also showed elevated intensity of MW-S compared to BPD and comorbid ADHD+BPD. We found no MW-S instability differences amongst clinical cases as well as cases versus controls. Negative content of MW-S was higher in BPD and comorbid ADHD+BPD compared to controls, with no differences between ADHD and controls. When controlling for depression/anxiety, the differences between BPD and comorbid ADHD+BPD and controls dissipated. MW-S is a trans-diagnostic process present in both ADHD and BPD. ... mehrYet, the underlying mechanisms of this experience may be driven by anxiety/depression in BPD but reflect a core process in ADHD.