When he starts in the summer, Hedger will be in charge of setting the direction of M&C Saatchi, which has been without a chief executive since Tom Bazeley left in August last year.
Hedger will take an equity stake in the business alongside chief creative officer Justin Tindall, chairman Tim Duffy, chief strategy officer Gabrielle Bell and managing director Tom Firth.
Before FCB Inferno, Hedger spent eight years at Leo Burnett, where he worked alongside Tindall. He held a number of senior strategy roles at Leo Burnett and was its group managing director between 2012 and 2014. Hedger started his career at J Walter Thompson in 1995.
Moray MacLennan, global chief executive of M&C Saatchi, said: "We scoured the world and Giles is quite a brilliant guy. He’s someone that can energise the agency."
FCB Inferno has promoted executive planning director Vicki Holgate to chief strategy officer. Holgate was lead strategist on Sport England’s "This girl can" campaign.