Graham joined the agency, when it was known as Euro RSCG, in 2011 from Ogilvy & Mather where he was creative partner.
In 2008, Graham took over joint responsibility for creative output at Ogilvy & Mather following the departure of the then executive creative director Malcolm Poynton.
A statement from Havas said that Graham is "looking for other opportunities as well as the possibility of setting up a start-up". He leaves next month.
Mark Whelan, the UK chief creative officer at Havas Creative Group, said: "Alasdair has done a great job of creating ideas that work across territories. As we are changing our model, it makes sense for him to redeploy that rare talent."