The WPP agency was told of the decision on Monday afternoon as Justin Billingsley, the Orange UK brand marketing director, conducted a series of face-to-face meetings with the competing agencies.
As the "lead media agency", MEC will oversee planning and buying on brand activity across both online and offline media. It will also handle Orange's offline direct-response media.
Despite losing the online brand activity to MEC, i-level, Orange's incumbent digital media agency, retains digital direct response media.
MEC's appointment follows a final pitch against Vizeum and i-level. Initiative, the incumbent on the offline business, had earlier been knocked out of the process along with Havas' MPG.
Orange's new agency arrangements will be in place by January 2009. Billingsley said: "We are confident that our decision will drive better integration."