Background: Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by the accumulation of amyloid-β plaques and hyperphosphorylated tau tangles in the central nervous system (CNS). It is the most common form of dementia and presents spectrum of clinical symptoms, including memory loss, increasing confusion, personality changes, and progressive deterioration in language, spatial awareness, abstract reasoning, and but cognitive impairment is the hallmark. The purpose of this systematic review was to: (a) evaluate the current evidence of cognitive impairment in adult patients following head and neck cancer (HNC) surgical management; (b) assess whether an association exists between cognitive decline and surgical treatment in HNC patients; and (c) identify gaps in the literature for future research directions and clinical practice. Methods: We performed a systematic search of the PubMed, Embase, Google Scholar, and Medline databases to identify English-language publications (January 2001 to January 2025) that investigated cognitive impairment in adult patients with head and neck cancer (HNC). Study quality was assessed using the Newcastle–Ottawa Scale (NOS). The search and reporting processes adhered to the PRISMA 2020 guidelines for systematic reviews. Studies evaluating clinical cognitive outcomes in HNC patients using validated cognitive assessment tools were included. The primary outcomes of interest were measurement of cognition and quality of life following HNC treatment. Results: Twenty-three studies met the eligibility criteria, a total of 28,054 patients with HNC. The majority of studies evaluated cognitive impairment as part of quality-of-life outcomes. Three studies specifically reported an increased incidence of dementia following HNC surgery. Two studies assessed postoperative cognitive decline reported an incidence of cognitive deterioration ranging from 7% to 40%. Three studies investigated postoperative quality-of-life scores reported an incidence of severe cognitive dysfunction of 37%. Two large studies identified a significant association between the neck dissection and cognitive decline. One study reported a dementia incidence of 0.7 per 100 patient-years, with a cumulative incidence of 10.34% over 8.6 years (95% CI; p = 0.028). Bilateral supraomohyoid neck dissection (SOHND) and modified radical neck dissection (MRND) demonstrated significant cognitive decline (CI 95%, p = 0.016). Conclusions: HNC management is known to be associated with reduced quality of life; however, the relationship between neck dissection and cognitive impairment remains under-researched. Further high-quality studies are needed to explore the potential association between deep cervical lymph node dissection (dCLND) and the development of neurodegenerative conditions, including AD. Investigating cognitive outcomes across different types of neck dissection is essential to identify underlying mechanisms and to clarify whether surgical disruption of cervical lymphatic pathways contributes to postoperative or long-term neurocognitive decline and guide future clinical practice. Level of Evidence: Not gradable.