Background: Persistent mind-wandering processes are associated with negative emotions; however, mindful states exhibit an anticorrelation with mind-wandering patterns. The aim of this study is to investigate the mediating role of excessive mind wandering (EMW), rumination, and trait mindfulness between attention-deficit hyperactivity disorder (ADHD) symptoms and the severity of anxiety/depression in adults with ADHD. Methods: Medication-free 159 adults diagnosed with ADHD completed a test battery that included a sociodemographic form as well as the Adult ADHD Severity Rating Scale, Hospital Anxiety Depression Scale, Mind Excessively Wandering Scale, Ruminative Response Scale, and Freiburg Mindfulness Inventory after the diagnostic examination according to the Structured Clinical Interview for DSM-5–Clinician Version (SCID-5). Results: ADHD symptoms exhibited an indirect, but not a direct, association with the severity of anxiety and depression, mediated by increased excessive mind wandering and rumination, as well as decreased trait mindfulness. Limitations: This study was carried out within a clinical population using a cross-sectional design. While diagnostic assessments were conducted using SCID-5, psychiatric symptoms were evaluated through self-report measures. Conclusions: Coping with EMW and rumination and promoting mindfulness ability may be promising targets in treating comorbid conditions in adults with ADHD. These findings have highlighted the effects of mentation processes on depression and anxiety, which are common comorbidities with ADHD in adults; however, longitudinal studies are needed.