We estimate the effect of neighborhood disorder on dementia risk and identify cardiometabolic dysregulation as a mediating biological pathway. Using Health and Retirement Study (2006–2020), we show that exposure to visible neighborhood disorder is associated with higher risk of dementia (Hazard Ratio: 1.37; 95% CI: 1.08–1.74) and higher risk of cognitive impairment no dementia (CIND; HR: 1.50; 95% CI: 1.22–1.85) over a 14-year follow-up. Mediation analysis reveals that a composite cardiometabolic risk score - aggregating seven biomarkers spanning inflammatory, cardiovascular, and metabolic systems - accounts for approximately 16% of the total neighborhood disorder–dementia association and 19% of the neighborhood disorder–CIND association. These findings are robust to competing-risk regression for mortality, restriction to non-movers, age-at-onset restrictions, and exclusion of pandemic-year data. The findings suggest that community interventions that simultaneously reduce visible signs of neighborhood decay and address cardiometabolic risk may yield dementia-prevention dividends beyond what individual-level clinical strategies alone can achieve.