OBJECTIVE: To examine whether mindfulness-based cognitive therapy (MBCT) would enhance attenuated amplitudes of event-related potentials (ERPs) indexing performance monitoring biomarkers of attention-deficit/hyperactivity disorder (ADHD). METHODS: Fifty adult ADHD patients took part in a randomised controlled study investigating ERP and clinical measures pre-to-post MBCT. Twenty-six patients were randomly allocated to MBCT, 24 to a wait-list control. Main outcome measures included error processing (ERN, Pe), conflict monitoring (NoGo-N2), and inhibitory control (NoGo-P3) ERPs concomitant to a continuous performance task (CPT-X). Inattention and hyperactivity-impulsivity ADHD symptoms, psychological distress and social functioning, and mindfulness skills were also assessed. RESULTS: MBCT was associated with increased Pe and NoGo-P3 amplitudes, coinciding with reduced 'hyperactivity/impulsivity' and 'inattention' symptomatology. Specific to the MBCT; enhanced Pe amplitudes correlated with a decrease in hyperactivity/impulsivity symptoms and increased 'act-with-awareness' mindfulness skill, whereas, enhanced P3 correlated with amelioration in inattention symptoms. CONCLUSIONS: MBCT enhanced ERP amplitudes associated with motivational saliency and error awareness, leading to improved inhibitory regulation. SIGNIFICANCE: MBCT suggests having comparable modulation on performance monitoring ERP amplitudes as pharmacological treatments. Further study and development of MBCT as a treatment for ADHD is warranted, in addition to its potential scope for clinical applicability to broader defined externalising disorders and clinical problems associated with impairments of the prefrontal cortex.