Background: Monoamine oxidase type B (MAO-B) inhibitors exhibit neuroprotective effects in preclinical models of PD but clinical trials have failed to convincingly demonstrate disease modifying benefits in PD patients. Objective: To perform a secondary analysis of NET-PD LS1 to determine if longer duration of MAO-B inhibitor exposure was associated with less clinical decline. Methods: The primary outcome measure was the Global Outcome (GO), comprised of 5 measures: change from baseline in the Schwab and England (ADL) scale, the 39-item Parkinson's Disease Questionnaire (PDQ-39), the UPDRS Ambulatory Capacity Scale, the Symbol Digit Modalities Test, and the most recent Modified Rankin Scale. A linear mixed model was used to explore the association between the cumulative duration of MAO-B inhibitor exposure and the GO, adjusting for necessary factors and confounders. Associations between MAO-B inhibitor exposure and each of the five GO components were then studied individually. Results: 1616 participants comprised the analytic sample. Mean observation was 4.1 (SD = 1.4) years, and 784 (48.5) participants received an MAO-B inhibitor. The regression coefficient of cumulative duration of MAO-B inhibitor exposure (in years) on the GO was 0.0064 (SE = 0.002, p = 0.001). Significant associations between duration of MAO-B inhibitor exposure and less progression were observed for ADL (p < 0.001), Ambulatory Capacity (p < 0.001), and the Rankin (p = 0.002). Conclusions: Our analysis identified a significant association between longer duration of MAO-B inhibitor exposure and less clinical decline. These findings support the possibility that MAO-B inhibitors slow clinical disease progression and suggest that a definitive prospective trial should be considered.