(1) Background: This study compared anterior attentional–intentional system performance between three groups: Parkinson’s disease (PD) patients with normal cognition (PD-NC), with mild cognitive impairment (PD-MCI), and a comparison group (CG). It also evaluated the feasibility of the recruitment and study procedures; (2) Methods: From 45 participants recruited, 39 were allocated (mean age 65.31; 43.59% men) to PD-NC, PD-MCI and CG (13 per group). To assess attention, we used three tasks from the ROtman–Baycrest Battery for Investigating Attention: Simple Reaction time (RT), Choice RT, and Prepare RT. We conducted a mixed-model analysis of variance with a 3 (groups) × 4 (tasks) design to compare reaction times; (3) Results: PD-MCI had slower reaction times than PD-NC (p = 0.028) and the CG (p = 0.052); there was no difference between PD-NC and CG. PD-MCI might perform worse on monitoring tasks than PD-NC, Z = −1.68, p = 0.092. Nearly half the volunteers from the CG and 87% of all eligible patients were enrolled in the study and completed all neuropsychological procedures; (4) Conclusions: General cognitive decline appears related to partial deficits in energization and tends to impair attentional monitoring. Furthermore, PD-NC exhibited similar reaction times to the CG. Results from the feasibility study contributed to the definitive study.