The present study is the first so far in empirically testing the recent conceptual view that the number of chronic diseases may mediate between the build-up of cognitive reserve (e.g., by educational attainment and cognitive level of job) on the one hand and cognitive performance on the other. We assessed Psychometric tests on processing speed and verbal ability in 2812 older adults (mean age = 77.9 years) from Switzerland. Individuals were interviewed regarding their education, occupation, and chronic diseases. Results showed that higher educational attainment and higher cognitive level of job were significantly related to better performance in processing speed (rs >= .15, ps < .001) and verbal ability (rs >= .27, ps < .001). These relations were significantly mediated via the number of chronic diseases. Mediation effects of the relation of educational attainment to cognitive performance were 5.3% exerted indirectly (β = .01, p = .007) for processing speed and 1.5% exerted indirectly (β = .01, p = .014) for verbal ability. Mediation effects of the relation of cognitive level of job to cognitive performance were 7.3% exerted indirectly (β = .01, p = .004) for processing speed and 1.8% exerted indirectly (β = .01, p = .015) for verbal ability. In conclusion, individuals with higher educational attainment and higher cognitive level of job in early and midlife may suffer from fewer chronic diseases later in life. This may finally preserve their performance in verbal ability and processing speed in old age.