Introduction: Cognitive training (CT) offers a potential approach for dementia prevention and maintenance of cognitive function in older adults. Online delivery provides a cost-effective means of implementing CT compared with in-person interventions, with the potential of providing an effective public health intervention for risk reduction. Methods: A double-blind 6-month online randomized controlled trial in adults older than 50 randomized to General CT, Reasoning CT, or control. The primary outcome was instrumental activities of daily living (IADL) in adults older than 60. Secondary outcomes were reasoning, verbal short-term memory, spatial working memory, verbal learning (VL), and digit vigilance in adults older than 50. Secondary analyses were performed with a group defined as showing age-associated impairment in reasoning according to baseline scores in this domain. Results: A total of 2912 adults older than 60 (6742 > 50) participated. General and reasoning packages conferred benefit to IADL (P = .008, P = .011), reasoning (P < 0.0001, P < .0001), and VL (P = .007, P = .008) at 6 months. Benefit in reasoning was evident from 6 weeks. Other benefits developed over 6 months. Analysis of participants with age-associated impairment also showed the same pattern of benefit. A clear dose-response effect was seen. Conclusions: Online CT confers significant benefit to cognition and function in older adults, with benefit favoring the Reasoning package. Scale of benefit is comparable with in-person training, indicating its potential as a public health intervention. Impact on the group with age-associated impairment indicates a particular sensitivity to this at-risk group, which merits further investigation. © 2015 AMDA - The Society for Post-Acute and Long-Term Care Medicine.