Iain Bundred served as the former PM's political spokesman throughout the 2010 general election, having been appointed to the role in 2009.
Now 30, Bundred was the youngest member of Brown's inner circle and previously worked as a special adviser to James Purnell.
He will join Ogilvy's London office next week, overseeing the agency's integrated media offering including crisis media relations and political comms.
He will report to Ogilvy PR head of office Ross Cathcart, who said: 'As the right-hand man to Gordon Brown during the last election, there is nothing Iain doesn't know about media and managing a crisis.'
In a carefully drafted statement, Bundred said: 'As PR moves away from traditional silos and towards offering 360-degree brand reputation strategies, Ogilvy PR is able to give clients an integrated service that other PR firms couldn't dream of.'
Bundred recently worked for Jordan's Prime Minister for six months.
The introductory agency was Portland.