This study aimed to assess the associations of both serum vitamin D status and supplementation with all-cause dementia, Alzheimer’s disease (AD), and vascular dementia (VD) incidence. It highlighted consistent associations between various facets of vitamin D and multivitamin intake, objectively measured vitamin D deficiency and insufficiency from blood samples, and 14-year dementia incidence in a study population aged 55 to 69 years at baseline. Subgroup analyses revealed effect modification by skin color with associations only observed in the non-brown/non-black skin color group and stronger effect estimates for vitamin D supplementation in younger compared to older study participants. Although results are encouraging and suggest a potential role for vitamin D supplementation in dementia prevention, particularly for those with vitamin D deficiency, caution is advised due to the observational nature of this study. RCTs with long follow-up periods are indispensable to establishing the efficacy of dementia prevention strategies.