Abstract Subjective cognitive decline (SCD), mild cognitive impairment (MCI), or severe Alzheimer’s disease stages are still lacking clear electrophysiological correlates. In 178 individuals (119 SCD, 40 MCI, and 19 healthy subjects (HS)), we analysed event-related potentials recorded during a sustained visual attention task, aiming to distinguish biomarkers associated with clinical conditions and task performance. We observed condition-specific anomalies in event-related potentials (ERPs) during visual encoding (P1/N1/P2) and decision-making (P300/P600/P900): SCD individuals showed attenuated dynamics compared to HS, while MCI individuals showed amplified dynamics, except for P300, which matched clinical severity. ERP features confirmed a non-monotonic trend, with MCI showing higher neural resource recruitment. Moreover, task performance correlated with condition-specific ERP gain and latencies across early and late ERP components. These findings enhanced the understanding of the neural mechanisms underlying cognitive decline in SCD and MCI and suggested potential biomarkers for early diagnosis and intervention. Highlights In encoding (P1/N1/P2) and decision (P600/P900) ERPs, SCD individuals showed attenuated dynamics compared to HS, while MCI individuals exhibited amplified dynamics compared to SCD. P300 dynamics matched clinical severity. MCI individuals demonstrated higher recruitment of neural resources, indicating a non-monotonic trend in ERP features between clinical conditions. Task performance correlated with condition-specific gain and latencies across multiple ERP components.