In this large-scale prospective epidemiological study, we examined associations of long-term adherence to the Mediterranean diet (MeDi) and subsequent cognitive function and decline. We included 16,058 women from the Nurses' Health Study, aged ≥70 y, who underwent cognitive testing by telephone 4 times during 6 y, beginning in 1995–2001, and provided repeated information on diet between 1984 and the first cognitive exam. Primary outcomes were the Telephone Interview for Cognitive Status (TICS) and composite scores of verbal memory and global cognition. MeDi adherence was based on intakes of: vegetables, fruits, legumes, whole grains, nuts, fish, red and processed meats, moderate alcohol, and the ratio of monounsaturated:saturated fat. Long-term MeDi exposure was estimated by averaging all repeated measures of diet (>13 y, on average). In primary analyses of cognitive change, the MeDi was not associated with decline in global cognition or verbal memory. In a secondary approach examining cognitive status in older age, determined by averaging all 4 repeated measures of cognition, each higher quintile of long-term MeDi score was linearly associated with better multivariable-adjusted mean cognitive scores differences in mean Z-scores between extreme quintiles of MeDi = 0.06 (95% CI: 0.01, 0.11); = 0.05 (95% CI: 0.01, 0.08); and = 0.06 (95% CI: 0.03, 0.10) standard units; P-trends = 0.004, 0.002, and <0.001 for TICS, global cognition, and verbal memory, respectively. These associations were similar to those observed in women 1–1.5 y apart in age. In summary, long-term MeDi adherence was related to moderately better cognition but not with cognitive change in this very large cohort of older women.