The study of nearly 200,000 people showed the risk fell by up to a third.
The team at the University of Exeter said the results were exciting, empowering and showed people were not doomed to get dementia.
The findings were revealed at the Alzheimer's Association International Conference.
What counts as a healthy lifestyle?
The researchers gave people a healthy lifestyle score based on a combination of exercise, diet, alcohol and smoking.
This is an example of someone who scored well:
- Doesn't currently smoke
- Cycles at normal pace for two-and-a-half hours a week
- Eats a balanced diet that includes more than three portions of fruit and vegetables a day, eats fish twice a week and rarely eats processed meat
- Drinks up to one pint of beer a day
And an unhealthy one?
- Currently smokes regularly
- Does no regular exercise
- Eats a diet that includes less than three servings of fruit and vegetables a week, and includes two or more servings of processed meat and of red meat a week
- Drinks at least three pints of beer a day