Standard mental functioning tests at the end of two years showed that participants randomly allocated to the programme had 25% better test scores than those receiving regular health advice.
For some tests, differences between groups were more striking. For executive functioning - the brain's ability to organise and regulate thought processes - scores were 83% higher in the programme group and for processing speed 150% higher.
Lead researcher Professor Miia Kivipelto, from the Karolinska Institute in Stockholm, Sweden, said: "Much previous research has shown that there are links between cognitive decline in older people and factors such as diet, heart health, and fitness. However, our study is the first large randomised controlled trial to show that an intensive programme aimed at addressing these risk factors might be able to prevent cognitive decline in elderly people who are at risk of dementia."
Findings from the Finnish Geriatric Intervention Study to Prevent Cognitive Impairment and Disability (Finger) are reported in the The Lancet medical journal.
Dr Simon Ridley, from Alzheimer's Research UK, said: "This study is one of the first of its kind, testing the benefits of a group of intensive lifestyle and health interventions on memory and thinking in older people at higher risk of dementia. "The initial results are promising and suggest that a combination of improving cardiovascular health and keeping mentally active could slow decline in some aspects of our thinking, but it's unclear which of the interventions carried the greatest benefit.
"Benefits on memory were not so clear from this study and we await the findings from the longer follow-up period to see whether this intervention also has long-term benefits in reducing the risk of dementia.
"We know that dementia is caused by a complex mixture of age combined with genetic and lifestyle risk factors. Further studies like this will be vital to help us unpick the best approaches to maintaining brain health as we age and potentially helping to reduce the burden of dementia in society."