Hare started at the UK media agency, when it was known as Walker Media, on its first day in 1997.
Phil Georgiadis, the global chairman of Optimedia Blue 449, said Hare will be responsible for ensuring the agency has "a consistent and strong identity" in every market.
"She has been a hugely important part of the story of the agency from the early days working on British Airways to her work on our first, full-service win (Channel 5) and her part on the Barclays’ pitch," he added.
Hare has most recently been vice-chairman of Blue 449 UK, which won Asda’s estimated £95m account in partnership with sister creative agency Saatchi & Saatchi in a double coup earlier this year.
Publicis Groupe, which bought a 75% stake in Blue 449 from M&C Saatchi in 2013, wants to turn the media agency into a global network as part of a group-wide reorganisation.
The French-owned holding company has moved from two media-buying groups, Starcom MediaVest and ZenithOptimedia, to four agencies, Starcom, MediaVest, Zenith and Optimedia Blue 449, which are all part of a single division, Publicis Media.
Optimedia Blue 449, which still operates in the UK under its original name of Blue 449, already has offices in at least 16 countries.