Brown, 33, will also take responsibility for business development for the Y&R/ Wunderman Alliance EMEA, the initiative introduced last year to deliver joint communications solutions by the two networks.
Y&R and Wunderman will continue to operate as separate business development units, but will each be overseen by Brown. Brown will report to William Eccleshare and Stewart Pearson, the chairmen and chief executives of Y&R EMEA and Wunderman EMEA respectively.
Eccleshare said: "Marcus succeeds through a combination of a profound marketing intellect and a rigorously disciplined approach. I'm certain he will flourish on a broader stage."