The move is one of a number of promotions, which also see the operations director, Toby Jenner, and the head of strategy, Steve Hatch, become joint managing directors.
In addition, Stuart Sullivan-Martin, the director of strategy, fills the newly created role of chief strategy officer.
The appointments follow a strong new-business run for MEC, which culminated in the capture of Danone's £28 million media planning and buying business last month. Other 2006 media wins include Nintendo, The Famous Grouse and Coca-Cola Enterprises.
George said: "This year has been a phenomenal one for MEC, building on a very strong 2005.
"The restructure of the management team, together with the senior appointments we have made and will continue to make, is designed to further strengthen our positioning for 2007 onwards.
"The fact that we've been able to promote from within shows the strength and depth of the agency board and indeed of the whole agency."
George joined MEC in 2004 after a 15-year career at ZenithOptimedia, where he rose to deputy managing director. Before this, George had a two-year spell at DFS Dorland, which he joined in 1986 as a media executive.