© 2019 Background: There is consistent evidence of an association between physical activity and walking performance in persons with multiple sclerosis (MS). To date, this relationship has been predominantly examined in young and middle-aged adults rather than in the rapidly-growing population of older adults with MS who often times have greater walking problems and are less physically active. This study examined whether physical activity was differentially associated with walking performance across three age groups of young (20–39 years), middle-aged (40–59 years), and older (60–79 years) adults with MS. Methods: The sample included 124 persons with MS who attended one testing session and provided demographic information, completed the Timed 25-Foot Walk (T25FW) and the Six Minute Walk (6MW) as measures of walking speed and walking endurance, respectively, and wore an accelerometer for a 7-day period. Results: Trend analysis indicated light physical activity did not significantly differ with increasing age; however, moderate-to-vigorous physical activity (MVPA), walking speed, and walking endurance declined with increasing age. Partial spearman's rank-order correlations between physical activity and walking outcomes that controlled for disease duration, race, and ambulatory disability within each age group indicated that the relationship between MVPA and walking performance was strong among older adults with MS (prs for MVPA and T25FW: young = –0.01, middle-aged = –0.16, older = –0.63*; prs for MVPA and 6MW: young = 0.10, middle-aged = 0.08, older = 0.68*). Conclusion: Interventions targeting MVPA may be an appropriate approach for managing walking impairment, particularly in older adults with MS.