•Reduced frontal P300 may precede cognitive and structural changes in cognitively normal Parkinson's disease (PD).•The reduction in P300 amplitude further spreads to centro-parietal areas in PD with mild cognitive impairment (PD-MCI).•PD-MCI demonstrated reduced putamen volume, which was associated with impaired executive function. Functional and structural brain alterations of cognitively normal Parkinson’s disease (PD-CN) and Parkinson’s disease mild cognitive impairment (PD-MCI) patients were investigated using event-related potentials (ERP) P300 and volumetric magnetic resonance imaging (MRI) parameters. Twenty three patients with PD-CN, 21 with PD-MCI, and 23 demographically-matched healthy controls were included. EEGs were recorded using a visual oddball task and mean amplitude and peak latency values of P300 were measured. Gray matter volumes (GMV) of thalamus, caudate, putamen, globus pallidus, hippocampus, amygdala and nucleus accumbens were obtained using FMRIB Integrated Registration and Segmentation Tool. Correlations among P300, subcortical GMV and cognitive performances were assessed. PD-CN patients demonstrated reduced P300 amplitudes compared to healthy controls. PD-MCI patients had lower P300 amplitudes than both PD-CN patients and controls and reduced volumes of the putamen compared to controls. Both putamen volumes and P300 amplitudes showed moderate associations with executive functions. Our findings support that P300 amplitude may be a useful marker for the detection of preclinical changes before the appearance of cognitive and structural deterioration in PD, as shown by decreased frontal P300 amplitudes in PD-CN. The reduction further spread to centro-parietal areas in PD-MCI patients, which was accompanied by lower putamen volumes. This study is the first to report on changes in ERP P300 amplitude and subcortical volume in well-matched samples of PD-CN, PD-MCI and healthy controls.