Whitmey, who was chief executive of Havas Helia for six years, quit last year to run Tesco’s customer loyalty marketing team.
Rees will take on responsibilities for Havas Helia from the end of January.
Havas UK confirmed the management structure beneath Rees will remain the same, with teams in Helia’s offices in London and Cirencester reporting into him as they did under Whitmey.
The agency told Campaign in November that "various options" were being looked at for Whitmey’s replacement, including hiring a replacement chief executive.
Havas added that the move was "testament to Havas’ unique Village structure and the closeness of the two agencies". The agencies work together on shared client accounts for Heathrow, Heathrow Express, Unilever and GlaxoSmithKline.
Rees joined Havas London in 2016, reporting to Europe chief executive and UK chairman Chris Hirst, after six years at Adam & Eve/DDB, where he was latterly group managing director and ran the Diageo and Volkswagen businesses.
He has also worked at AIS London, Wunderman and Bartle Bogle Hegarty and began his career on the client side as a product manager at Boots in 1994.
Hirst said: "Havas London has seen a huge upturn in its trajectory since Xav and his team took over.
"Combined with his experience across a number of different specialisms, and the fact this appointment will encourage even greater integration between our agencies, I honestly can’t think of anyone better for the Helia job."