OBJECTIVE: Cognitive reserve is associated with a lower risk of dementia, but the extent to which it shapes cognitive aging trajectories remains unclear. Our objective is to examine the impact of 3 markers of reserve from different points in the life course on cognitive function and decline in late adulthood. METHODS: Data are from 5,234 men and 2,220 women, mean age 56 years (standard deviation = 6) at baseline, from the Whitehall II cohort study. Memory, reasoning, vocabulary, and phonemic and semantic fluency were assessed 3× over 10 years. Linear mixed models were used to assess the association between markers of reserve (height, education, and occupation) and cognitive decline, using the 5 cognitive tests and a global cognitive score composed of these tests. RESULTS: All 3 reserve measures were associated with baseline cognitive function; the strongest associations were with occupation and the weakest with height. All cognitive functions except vocabulary declined over the 10-year follow-up period. On the global cognitive test, there was greater decline in the high occupation group (-0.27; 95% confidence interval [CI], -0.28 to -0.26) compared to the intermediate (-0.23; 95% CI, -0.25 to -0.22) and low groups (-0.21; 95% CI, -0.24 to -0.19); p = 0.001. The decline in reserve groups defined by education (p = 0.82) and height (p = 0.55) was similar. INTERPRETATION: Cognitive performance over the adult life course was remarkably higher in the high reserve groups. However, rate of cognitive decline did not differ between reserve groups with the exception of occupation, where there was some evidence of greater decline in the high occupation group.