Objective To investigate the mediating role of serum vitamin D ₃ levels in the relationship between depression severity and cognitive function among elderly individuals with mild cognitive impairment (MCI). Methods A total of 477 elderly individuals with MCI who met the inclusion criteria were recruited via convenience sampling from the Health Service Center of Matang Street Community, Yijiang District, Wuhu City, Anhui Province, between January 2023 and December 2023. Data were collected using a general questionnaire, the Geriatric Depression Scale, and the Mini-Mental State Examination. Serum vitamin D ₃ levels were measured for all participants. Pearson correlation analysis was conducted using SPSS 27.0, and the mediating effect was analyzed using Model 4 in the Process plugin. Results Cognitive function in elderly individuals with MCI was negatively correlated with depression severity (P<0.001) and positively correlated with serum vitamin D ₃ levels (P<0.05). Bootstrap test results indicated that serum vitamin D ₃ levels partially mediated the relationship between depression and cognitive function in elderly individuals with MCI, with a mediating effect of -0.019 (95% CI: -0.038 to -0.005), accounting for 8.02% of the total effect. Conclusion Serum vitamin D ₃ levels play a partial mediating role in the relationship between depression and cognitive function in elderly individuals with MCI.