The pair will oversee all of the agency's main accounts, with a particular focus on Microsoft and Cancer Research UK.
They join Tom Powell and Graham Jenks, the creative directors on Ford, who have also been promoted to creative group heads after two years at the agency.
The group heads will all report directly to David Harris, executive creative director of the WPP-owned agency.
During their time at Publicis-owned Digitas, Tillinghast and Barrie worked as creative directors on accounts including Vauxhall, HP and Samsung, winning awards at Cannes Lions and Campaign Digital and Press.
Prior to joining the agency Barrie had worked at Modem Media, and was one of the founding members of Publicis Modem.
Tillinghast had previously worked at Ogilvy, McCann Erickson and Archibald Ingall Stretton.
Harris said: "I'm excited that our new group head line-up will push our creative work to a higher level and help nurture the talent we have been growing over the last couple of years."