As the population ages, risks for cognitive decline threaten independence and quality of life of older adults. Classically, psychological assessment tools that evaluate cognitive functioning are administered in face-to-face laboratory sessions, which are time- and resource-consuming. The present study set out to examine whether the eCOGTEL—an online adaptation of the Cognitive Telephone Screening Instrument (COGTEL; Kliegel et al., 2007)—represents a reliable measure of cognitive performance in adulthood. Therefore, an age-stratified adult lifespan sample of 253 participants (aged 19–86 years) completed a face-to-face assessment in the laboratory and a self-administered online version, at their homes. A second, independent sample of 176 younger adults (aged 19–30 years) performed a test–retest assessment of the eCOGTEL. Results showed strong correlations between overall cognitive scores assessed online and in the laboratory, as well as a high test–retest reliability. Further, comparable data distributions between both assessment modes underline the feasibility of the eCOGTEL across the adult lifespan and particularly in older age. Our findings thereby indicate that the eCOGTEL can reliably measure cognitive performance across the lifespan at reduced costs, which may help detecting individuals at risk of developing age-related cognitive decline. Due to these strengths, the eCOGTEL represents a valuable contemporary approach for the resource-efficient online assessment of cognition, which may benefit a broad array of fundamental and applied research fields, such as clinical and organizational psychology.