A simple scan currently available at NHS opticians could identify the thousands of people at greater risk of developing the condition.
Experts hope targeting such people with drugs and lifestyle changes could slow or even stop the onset of dementia.
One of the world’s leading eye specialists used data from optical scans on 32,000 Britons aged 40 to 69 which measured the thickness of a layer of neurons on the retina.
Those with thin retinas were twice as likely to perform poorly in tests on everyday memory, reaction time and reasoning.
Researchers at University College London (UCL) and Moorfields Eye Hospital found that when tested again three years later these people were twice as likely to have suffered mental decline.
Separately, researchers in the Netherlands have published a paper building on the findings which show these changes can be tracked to predict full blown dementia a decade later.
Lead author Prof Paul Foster said: “The combination of the two studies showing the increased risk does put it beyond doubt.
“There is unquestionably a link between changes in the retina and changes in people’s mental state.”
The UCL research paper could revolutionise treatments of the devastating disease that affects 850,000 people in the UK.
This is expected to increase to a million sufferers by 2025.