One-carbon metabolism is a fundamental intracellular pathway responsible for methyl donor transfer, governing essential processes such as epigenetic regulation and neurotransmitter synthesis, and thus plays a pivotal role in brain health and disease. Dysregulation of this metabolic pathway is increasingly linked to cognitive disorders like Alzheimer's disease and vascular cognitive impairment, where it contributes to pathology by disrupting epigenetic programming, neuroenergetics, and synaptic transmission. This review synthesizes current evidence to elucidate the mechanistic connections between one-carbon metabolism and cognitive function, focusing on its metabolic pathways, key pathogenic mediators, such as homocysteine, and potential therapeutic interventions targeting these pathways.