In a letter to Health Secretary Jeremy Hunt, they said persuading people to eat fresh fruits and vegetables, nuts, fish and olive oil was "possibly the best strategy currently available".
But it was being largely ignored because of the "low awareness and prestige given to diet by many in the medical profession", they warned, calling for investment in an education programme.
Story continues below the slideshow...
Dementia experts from G8 countries will gather in London this week for a meeting convened by David Cameron as part of the UK's presidency of the group of leading economies.
Mr Hunt has called dementia a health and care "time bomb" with the number of people living with the condition expected to triple worldwide to 135 million by 2050, according to a recent report.
There is also a lack of diagnosis in England and Wales - with fewer that half of cases formally recognised by GPs and patchy performance across different areas.
Critics are also concerned about high levels of anti-psychotic drug prescription.
Among signatories to the letter were former chair of the Royal College of General Practitioners, Professor Clare Gerada, the chair of the National Obesity Forum, Professor David Haslam, Professor of Clinical Epidemiology at the University of Liverpool Simon Capewell and London cardiologist Dr Aseem Malhotra.
They said successfully encouraging people onto a healthier diet could have a "far greater impact in the fight to reduce the dramatic increasing rates of the disease than pharmaceutical and medical interventions" than the "dubious benefit of most drugs".
It can also protect against coronary heart disease, hypertension and diabetes.
Research by the University of Exeter's Medical School found a majority of studies suggested the diet could improve cognitive function, lower rates of decline and reduce the risk of Alzheimer's disease.
However, results for mild cognitive impairment - the stage before Alzheimer's or dementia, when someone could be experiencing some cognitive difficulties - were inconsistent.
Dr Malhotra said. "The evidence base for the Mediterranean diet, in preventing all of the chronic diseases that is plaguing the western world is overwhelming.
"This includes cardiovascular disease, type 2 diabetes, Alzheimer's and cancer.
"Policy makers and the public need to know that such a diet is far more potent than the often dubious benefit of many medications and without side effects."
Dr Simon Poole, a leading advocate of the Mediterranean diet who organised the letter, said: "Educating all generations, including our children, in the importance of a good diet in maintaining health in old age is a project which will take years, but is absolutely essential.
"We are calling upon policy makers to not only support the care and treatment of those who are already suffering from dementia, but to make significant investments in work which will see benefits beyond the period of one or two parliaments."
A Department of Health spokesman said: "Eating a healthy, balanced diet is important for everyone to cut the risk of dementia as well as conditions like heart disease, stroke and diabetes.
"The G8 summit this week will provide a unique opportunity to make real progress much faster, and re-double international efforts to find effective treatments and cures for dementia.
"We are already taking the lead in helping people stay healthier for longer. Our Change4Life programme continues to support families in making healthier lifestyle choices."