The An Education star was invited to meet with the politician at 10 Downing Street on Monday (10Dec12) for the Alzheimer's Society's new awareness drive, Christmas to Remember, which calls on families to keep an eye out for dementia symptoms while spending the holidays with elderly relatives.
Mulligan and Cameron swapped their favourite festive memories as part of the promotional campaign, and the actress has urged her fans to share their own beloved Christmas tales.
She says, ""My favourite Christmas memory is every Christmas we spent with my grandmother. She used to alternate between us and our cousins and every year we were lucky enough to have her was particularly precious.
""This Christmas we want people to tell us about their favourite Christmas memory - it can be funny, sad, silly or serious.""