This study aimed to identify CSF and plasma proteins that mediate the association between age and mild cognitive impairment (MCI) and Alzheimer's disease using mediation analysis. By focusing on proteins significantly associated in both CSF and plasma, we sought to identify biomarkers accessible for clinical applications. Proteomic measurements were obtained from CSF and plasma from a cohort of cognitively normal and MCI patients at the Johns Hopkins Alzheimer's Disease Research Center using Olink Proximity Extension Assay technology. Mediation effects were estimated using single- and multiple-mediator models and validated in three independent datasets: Duke (CSF), ADNI (CSF), and UK Biobank (plasma). Over 3000 proteins in 86 patients were analyzed. Three candidates, leiomodin-1 (LMOD1), glial fibrillary acidic protein (GFAP), and elastin (ELN), met the criteria for mediation in both CSF and plasma. Multiple mediator models demonstrated a significant combined mediation effect on MCI in CSF (OR: 1.122, 95 % CI: 1.026-1.439) and plasma (OR: 1.142, 95 % CI: 1.058-1.410). Across validation cohorts, GFAP consistently showed significant mediation effects (Duke CSF: OR: 1.114, 95 % CI: 1.069-1.206; ADNI: OR: 1.004, 95 % CI: 1.000-1.009; UK Biobank: OR: 1.030, 95 % CI: 1.026-1.034). In contrast, ELN and LMOD1 demonstrated mediation effects in the discovery dataset but were not consistently reproduced in external cohorts. Our findings highlight GFAP as a robust mediator of age-related risk of cognitive impairment across CSF and plasma, supporting its utility as a practical biomarker. ELN and LMOD1 may represent exploratory candidates reflecting extracellular matrix and vascular processes requiring further validation.