Ryan, who will report to the Ogilvy & Mather UK chairman, Paul O'Donnell, joins from Ogilvy Public Relations, where he was the social lead of the WPP Ford team.
Social@Ogilvy launched earlier this month and aims to integrate social media with other disciplines in the group, working across the agency's range of clients such as IBM, Nestlé, Unilever and UPS.
Before Ogilvy, Ryan spent a year at DraftFCB developing its social media insights programme. In 2006, he was one of the founding partners of RMM, a specialist digital and social media planning agency. He worked as a digital planner on the award-winning Sony Bravia "balls" campaign while at Tonic.
O'Donnell said: "Leo understands the strategic importance of integrating the right digital elements and social technologies for clients."