We investigated whether the relations of the cognitive reserve markers education and cognitive level of work to the physical and the mental component of health-related quality of life (HRQoL) were mediated via cognitive functioning in a large sample of 701 older adults (mean age = 70.36 years, SD = 6.87). HRQoL was measured with the 12-item Short-Form Health Survey. To measure cognitive functioning, we used the COGTEL, which is a cognitive test battery assessing prospective memory, verbal short-term memory, verbal long-term memory, working memory, verbal fluency, and inductive reasoning, combined into a composite score. Individuals reported information on their education and their main profession during their working life. Mediation analyses showed that better cognitive functioning mediated the association of higher values in the cognitive reserve markers (longer education and a high cognitive level of work) with higher HRQoL. In conclusion, the present study suggests that the sustainability of cognitive functioning in old age with the help of cognitive reserve is fundamental for the sustainability of quality of life, presumably particularly for the mental HRQoL component.