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: