Illnesses such as heart disease, high blood pressure, diabetes, heart failure, depression, liver disease, stroke, arthritis and cancer all impact on the risk of mental decline, the study found.
Susceptibility is greater when these sorts of conditions develop at a younger age (mid-50s), rather than later in life.
Younger age onset for the conditions up to the age 70 also pushed the risk of dementia up by 18 percent.
Experts, including representatives from University College London (UCL), said it highlights how prevention and management of chronic illness in adulthood could reduce vulnerability to developing dementia in later life.
They added: “Given the lack of effective treatment for dementia and its personal and societal implications, finding targets for prevention is imperative.”
For the study, published in the BMJ medical journal, experts looked at those having two or more conditions when people were aged 55, 60, 65 and 70.
From the 10,095 British men and women taking part in the Whitehall II Study, from 1985 to 1988, seven percent had two or more serious illnesses aged 55, rising to 32 percent at age 70.
Overall, 639 cases of dementia occurred over a typical 32-year followup.
After adjusting for factors such as socio-economic status, diet and lifestyle, having two or more conditions aged 55 pushed up the risk of dementia almost two-and-a-half times, compared to people with none.
Meanwhile, developing two or more conditions between the ages of 60 and 65 was associated with a 1.5-fold higher chance.
For those with three or more chronic conditions aged 55, there was around a five-fold higher likelihood of dementia.
The risk fell dramatically if people were aged 70 before they developed chronic health conditions.
All the people in the research were aged 35 to 55 at the start of the Whitehall II Study.
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