In this narrative review, we address mild cognitive impairment, a frequent complication of Parkinson's disease (PD) and atypical parkinsonian disorders (APDs). Recent diagnostic criteria have blurred the lines between PD and dementia with Lewy bodies (DLB), particularly in the cognitive domain. Additionally, atypical parkinsonian syndromes like progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) often present with significant cognitive decline. Even multiple system atrophy (MSA) can be associated with cognitive impairment in some cases. Several biomarkers, including imaging techniques, such brain magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography (FDG-PET), as well as pathological proteins either of the cerebrospinal fluid (CSF), such as Tau, amyloid beta, and synuclein, or of the serum, such as neurofilament light chain (Nfl) are more and more often utilized in the early differential diagnosis of APDs. The complex interplay between these conditions and the evolving understanding of their underlying pathologies highlight the need for further research to refine diagnostic criteria, possibly incorporate the new findings from the biomarker's field into the diagnostic criteria and develop targeted therapeutic strategies.