•First meta-analysis quantitatively summarizing relevant literature on cognitive event-related potentials (ERPs) in ADHD across the lifespan.•ADHD is associated with alterations in neurophysiological functioning during cognitive tasks.•On a group level, individuals with ADHD show moderate deviance mainly regarding later cognitive ERPs (P300, CNV, Pe).•These later cognitive ERPs represent a range of potential ADHD biomarkers.•Substantial heterogeneity and medium effect sizes (d < 0.6) limit the use of ERPs for clinical practice on an individual level. The current meta-analysis summarizes relevant literature on earlier (P100, N100, P200, N200, ERN/Ne) versus later (P300, Pe, CNV) cognitive Event-Related Potential (ERP) differences between children, adolescents, and adults with Attention-Deficit/Hyperactivity Disorder (ADHD) and without ADHD (non-ADHD). Furthermore, the heterogeneity in previous research is addressed by analyzing potentially relevant demographic and methodological moderators (age group, IQ, medication, comorbidity, task, cognitive function, modality, inter-stimulus-interval, number of electrodes). Via database search 52 relevant articles were identified including n = 1576 ADHD and n = 1794 non-ADHD. Using multilevel-models, pooled effect sizes were calculated. For earlier components, individuals with ADHD showed shorter Go-P100-latencies than non-ADHD. For later ERPs, individuals with ADHD showed smaller Cue-P300-amplitudes, longer Go-P300-latencies, smaller NoGo-P300-amplitudes, longer NoGo-P300-latencies, smaller CNV-amplitudes, and smaller Pe-amplitudes. The substantial heterogeneity identified for most of the ERP components could be explained by the demographic and methodological moderators of interest. This meta-analysis identified relevant moderate group differences (−0.32<d<−0.57), mainly regarding later cognitive ERPs. Nevertheless, results are characterized by substantial heterogeneity and the moderate effect sizes (d<0.6) limit the use for clinical application.