Frontal subcortical cognitive defects are predominant in Parkinson’s disease (PD). Temporal lobe dysfunction seems more relevant for progression to dementia. We aimed to study the relative importance of temporal lobe defects versus executive impairment in the progression to dementia in PD by using proton magnetic resonance spectroscopy (¹H-MRS). The ¹H-MRS features of PD patients with intact cognition (PD-CgInt; n = 16), mild cognitive impairment (MCI; n = 15) and dementia (PDD; n = 15) were compared, to delineate the metabolic alterations correlating with cognitive status. Metabolite concentrations were acquired from voxels localized to the hippocampus and dorsolateral prefrontal cortex (DL-PFC). Cognitive status was established following the Movement Disorder Society PDD criteria, administering the Clinical Dementia Rating Scale and Mattis Dementia Rating Scale. The Parkinson’s Disease Cognitive Rating Scale (PD-CRS) was used to correlate ¹H-MRS with neuropsychology. N-acetylaspartate (NAA) concentrations in the right DL-PFC were decreased in PD-MCI compared with PD-CgInt patients (p = 0.002), and correlated with frontal subcortical tasks. Decreased NAA concentrations in the left hippocampus in PDD compared to PD-MCI (p = 0.03) correlated with confrontation naming. The present findings support that executive impairment is related to dorsolateral prefrontal dysfunction from the early stages, while progression to dementia is linked to the additional impairment of temporal lobe structures. The PD-CRS was able to capture the differential impairment of prefrontal versus temporal cortical areas.