Background: Parkinson’s disease and dementia are major neurodegenerative disorders that substantially contribute to disability, dependency, and mortality worldwide. Although prior CDC WONDER studies have separately evaluated Parkinson’s disease and dementia mortality trends, fewer analyses have examined national mortality patterns in which both conditions are recorded on death certificates simultaneously over extended time periods. Methods: We analyzed U.S. death certificates from 1999 through 2025 using the CDC WONDER Multiple Cause of Death database, identifying deaths among adults aged ≥45 years in which both Parkinson’s disease (ICD-10 G20) and dementia-related codes (F01, F03, G30, G31) were recorded anywhere on the certificate. This operational definition captures co-recorded diagnoses and does not identify clinically confirmed Parkinson’s disease dementia. Age-adjusted mortality rates (AAMRs) per 100,000 were standardized to the 2000 U.S. standard population, a method that controls for shifts in population age structure over time and allows valid temporal comparisons independent of absolute population growth. Joinpoint regression was used to quantify trends. Sensitivity analyses excluded 2025 provisional data and the COVID-19 period (1999–2019). Results: A total of 337,721 deaths were identified. Overall AAMR increased from 5.75 (95% CI: 5.60–5.90) in 1999 to 11.15 (95% CI: 10.98–11.32) in 2025 (AAPC: 2.07; p = 0.002). A sharp transient increase occurred in 2020, attributable to pandemic-related factors including disproportionate COVID-19 mortality among older adults with neurodegenerative conditions, care disruptions, and changes in death-certificate coding practices. Following this pandemic-era peak, AAMRs declined significantly through 2025 and should be interpreted cautiously given provisional data. Males (AAPC: 2.14), non-Hispanic White individuals (AAPC: 2.29), the Midwest region (AAPC: 2.65), and non-metropolitan areas carried the highest mortality burden. Mortality was greatest among adults aged ≥85 years. Conclusion: Population-level death rates involving co-recorded Parkinson’s disease and dementia demonstrated significant temporal changes over the study period, with marked demographic and geographic disparities. These findings reflect death-certificate surveillance data and cannot establish clinical co-occurrence, causal relationships, or individual disease risk.