There is a complex interplay between mobility and cognition in older adults. We have previously shown that a high-DHA multi-nutrient supplement improves habitual walking speed, verbal memory and psychomotor response latency in older women. Exercise also improves mobility and cognition in older adults, and n-3 fatty acids and exercise share a range of overlapping biological effects. This study examined for the first time the effects of the high-DHA multi-nutrient supplement and aerobic exercise on mobility and cognition in older women. Women (mean age 67 (sd 8) years) were assigned to the following groups: multi-nutrient (1 g DHA, 160 mg EPA, 240 mg Ginkgo biloba, 60 mg phosphatidylserine, 20 mg d-α tocopherol, 1 mg folic acid and 20 µg vitamin B12 per d, n 13), multi-nutrient and exercise (spin class twice per week, n 14), exercise and placebo (n 12) or placebo (n 12). The multi-nutrient was given for 24 weeks and exercise for 12 weeks. No treatment effects were observed for the primary outcome, habitual walking speed. Improvements in verbal memory and executive function were seen for all treatments groups v. placebo (all, P < 0·05). Significant improvements in self-reported emotional well-being were seen with multi-nutrient and exercise groups v. placebo (P = 0·03). The results suggest that the high-DHA multi-nutrient supplement produces similar improvements in cognitive function to aerobic exercise, offering the intriguing prospect that supplementation may be able to mitigate some of the effects of low physical activity on cognitive function in the elderly.