Carey Mulligan has become the first UK Global Dementia Friends Ambassador.
The actress's new role will see her draw attention to the importance of making communities dementia-friendly.
It is hoped she will renew focus on the Alzheimer's Society's Dementia Friends programme.
Volunteers who take part in the scheme are able to attend free sessions to learn how to spot the signs of dementia and offer support for those affected. They can also watch educational videos online.
The Oscar-nominated actress has long been a supporter of the Alzheimer's Society charity. Her grandmother Margaret, known as "Nans", was diagnosed with Alzheimer's in 2004.
"My Nans has dementia and I have experienced first-hand how devastating it can be," Carey said.
"It affects everyone differently, and it's so important that everyone affected by the condition is treated with the respect and dignity that they deserve. At the moment, there's not nearly enough awareness and as a global society we have a duty to change that.
"The first step involves educating people and breaking down stigma - not just on our doorstep, but across the world.
"I've seen my mum doing this in her role as a Dementia Friends champion - now I'm honoured to become the first Global Dementia Friends Ambassador and help Alzheimer's Society and the Government change global attitudes towards dementia."
Health Secretary Jeremy Hunt said: "We have made great strides in improving diagnosis rates, investing in research and creating the first dementia-friendly communities, but we still have much further to go to promise everyone that they will be able to live well with the condition.
"The impact of dementia tears at families and at our social fabric - that's why making progress is a key Government priority. Carey Mulligan will be a great asset both in raising awareness and promoting the benefits of the Dementia Friends programme - at home and globally."
Jeremy Hughes, chief executive of Alzheimer's Society, added: "Dementia doesn't stop at UK borders - it's the biggest health crisis facing the world today.
"One of the biggest challenges in creating dementia-friendly communities is getting people to talk about dementia. Dementia Friends has proved to be a great springboard to inspire global dementia action. We are delighted that Alzheimer's Society supporter Carey Mulligan will be leading the way in her role as Global Dementia Friends Ambassador."
The announcement comes as the Government launched a new "Dementia Atlas" which aims to highlight best-performing areas on issues such as prevention, diagnosis and support.