© 2018 The Authors. Annals of Clinical and Translational Neurology published by Wiley Periodicals, Inc on behalf of American Neurological Association. Background and Objective: Hispanics with multiple sclerosis (MS) present younger and more often with optic neuritis (ON) as compared to Whites in the western United States. Regional differences related to Hispanic genetic admixture could be responsible. We investigated the association between global genetic ancestry and ON and age at onset of MS in Hispanics. Methods: Data were obtained for 1033 self-identified Hispanics with MS from four MS-based registries from four academic institutions across the United States January 2016–April 2017. Multivariate regression models, utilizing genetic ancestry estimates for Native American (NA), African, and European ancestry, were used to assess the relationship between genetic ancestry and ON presentation and age of MS onset, defined as age at first symptom. Results: Genetic ancestry and ON proportions varied by region where NA ancestry and ON proportions were highest among Hispanics in the southwestern United States (40% vs. 19% overall for NA and 38% vs. 25% overall for ON). A strong inverse correlation was observed between NA and European ancestry (r = −0.83). ON presentation was associated with younger age of onset (OR: 0.98; 95% CI: 0.96–0.99; P = 7.80 × 10−03) and increased NA ancestry (OR: 2.35 for the highest versus the lowest quartile of NA ancestry; 95% CI: 1.35–4.10; P = 2.60 × 10−03). Younger age of onset was found to be associated with a higher proportion NA (Beta: −5.58; P = 3.49 × 10−02) and African ancestry (Beta: −10.07; P = 1.39 × 10−03). Interpretation: Ethnic differences associated with genetic admixture could influence clinical presentation in Hispanics with MS; underscoring the importance of considering genetic substructure in future clinical, genetic, and epigenetic studies in Hispanics.