Progression from Parkinson's disease (PD) to Lewy body dementia is a major clinical concern. Although several progression-associated loci have been identified, their cumulative effects on cognitive decline have not been systematically evaluated. To assess the dose-dependent effect of five candidate progression loci linked to synaptic vulnerability (RIMS2, TMEM108, GBA1) and amyloid-tau pathology (APOE, WWOX), we analyzed 7745 participants from 24 cohorts with 28,737 longitudinal visits over 15 years using random-effects meta-analyses of cohort-specific Cox proportional hazards models. Dementia risk increased monotonically with the number of progression loci (0, 1, 2, or ≥3). A single locus conferred a 1.56-fold increase in risk (hazard ratio (HR) = 1.56, 95% CI: 1.28-1.89), rising to 3.21-fold for two loci (HR = 3.21, 95% CI: 2.19-4.70) and 7.49-fold for three or more loci (HR = 7.49, 95% CI: 4.98-11.28). Individually, GBA1 (HR = 2.09), APOE ε4 (HR = 1.71), RIMS2 (HR = 1.90), TMEM108 (HR = 2.05), and WWOX (HR = 1.56) were associated with dementia risk, but there was heterogeneity between clinical trials, biomarkers, and population-based cohorts. Multi-locus dosage increases dementia risk in a monotonic manner and may improve stratification and clinical trial design in PD.