The company has also promoted Robert LePlae, the global president of Arnold Worldwide, to the position of global chief executive of Arnold Worldwide.
Benett has been with Havas for nearly a decade and during the three years at his role at Arnold Worldwide, whose clients include Jack Daniel's, Unilever and Volvo, the agency grew by more than 30%
In this new role, Benett will be responsible for leading the day-to-day management of the global Havas Worldwide network.
LePlae joined Havas last year having spent 11 years at Omnicom-owned TBWA, where he rose to president of its North American division. Immediately prior to joining Arnold, he served as North American president at McCann Worldgroup.
Both men will report to David Jones, the global chief executive of Havas.
According to Jones, the appointments are a bid to further accelerate growth and innovation at the two companies, as Havas Worldwide is "coming off one of the best new business streaks in its history".
Jones said: "These senior management moves represent another important step in delivering our strategic plan."
The appointments come after Havas Worldwide, previously known as Euro RSCG, co-located its media, creative and digital teams into one building in both Paris and New York.