Robertson co-founded the UK-based not-for-profit One Young World with support from Havas back in 2009, with a mission to empower young leaders from around the world.
She will continue as a consultant to Havas through to the end of next year.
Over a 12-year career at Havas, Robertson has led the UK Havas Group in serving global clients including Barclays, Reckitt Benckiser and Jaguar, and most recently led the UK acquisition of Work Club in 2014.
Andrew Benett, the global chief executive of Havas Creative Group and Havas Worldwide, said Robertson has "contributed considerable value" to Havas over the years, and delivered record growth for the group last year.
He said: "Kate has an energy and enthusiasm for her work that is unparalleled."
Robertson said: "I’ve had a great time at Havas with brilliant clients and wonderful employees. But One Young World has become an overwhelming passion and one that needs an enormous amount of attention."
The forum will host its 2015 summit in Bangkok this November.
Robertson’s is the latest in a series of exits by long-term Havas employees. Chris Pinnington, the company’s global chief operating officer, left last July after 30 years and David Jones, the former chief executive of Havas Worldwide and co-founder of One Young World, quit in January 2014 after 15 years at the agency.
At the same time the network has made a string of high profile hirings, including Sean Lyons from R/GA who now leads Havas digital strategy and Chris Hirst, the former chief executive of Grey London who joins in September to lead Havas across Europe.