He replaces Fran Kelly, who is becoming vice-chairman.
In his new role, Benett will lead the Arnold micro-network, which has offices in New York, Boston, Washington DC, London, Madrid, Milan, Prague and Lisbon.
He will be based at the agency's head office in Boston and report to David Jones,
the global chief executive of Havas Worldwide and Euro RSCG Worldwide.
Benett has spent the past five years at Euro RSCG, where his remit included the role of global chief strategy officer across Euro's network of 233 offices.
Jones said: "I've worked very closely with Andrew since hiring him in 2004. He's strong strategically, has excellent creative judgment and has extensive experience working on major global brands."