Vallee leaves the role of managing director at George P Johnson (GPJ) after only four months, although he had been acting managing director since September 2012. Vallee first joined the agency in 1999 as account director.
In the interim, Joost Dop, Project Worldwide’s CEO for EMEA, will assume the responsibilities of managing director GPJ UK.
Vallee told C&IT: "It is time for me to do something different and pursue other opportunities within the events industry.
"I was born and bred at GPJ and have been there for 15 years in total. It is an amazing company with amazing people."
Dop added: "I would like to thank Olivier for his important contribution to the business over the past few years. We wish him well for the future.
"GPJ has a strong management team which is very capable of running the business in the interim while the search for a new MD is under way. Our strategy is to maintain our position as one of EMEA’s leading experience marketing agencies and this key management role is integral to those ambitions."