It remains unclear so far whether the role of cognitive reserve for cognitive functioning in old age may differ between individuals with low, compared to those with high functional fitness status. Therefore, the present study set out to investigate the relation of education and cognitive leisure activity as key markers of cognitive reserve to mini-mental state in old age (as an indicator of the extent of cognitive impairment) and its interplay with functional fitness status in a large sample of older adults. We assessed MMSE in 701 older adults (M = 70.4 years, SD = 6.9, range: 60–91). We measured functional fitness status using the Senior Fitness Test battery and interviewed individuals on their education and cognitive leisure activity. Results showed that better functional fitness status, longer education, and greater engagement in cognitive leisure activity were significantly related to higher MMSE scores. Moderation analyses showed that the relations of education and cognitive leisure activity to MMSE scores were significantly larger in individuals with low, compared to those with high functional fitness status. In conclusion, cognitive functioning in old age may more strongly depend on cognitive reserve accumulated during the life course in older adults with low, compared to those with high functional fitness status. These findings may be explained by cross-domain compensation effects in vulnerable individuals and may (at least partly) account for the large variability in cognitive reserve–cognition relations debated in the literature.