North joined CDP in 1991 and has been the managing director of the agency since it merged with Travissully in 2001. He has worked with clients such as Northern Foods, Bacardi, Canon and Scottish Courage.
In recent years, North has been instrumental in repositioning the Dentsu-owned agency as a slender outfit that can turn work around quickly.
"Simon has been the architect of much of our change and it's right that he should now take on the chief executive's role," Travis said.
North's next challenge will be to communicate this positioning to clients and build on the agency's glorious past.
North said: "It's been a tough time for the industry over the past two or three years - and in a small agency like ours, everybody has had to work their socks off.
"My challenge will be to keep everybody working hard and to continue to drive the growth we have experienced over the past nine months."
The agency has put in an encouraging new-business performance over the past year, winning briefs from Toshiba, NEC and Bacardi. However, it lost the Yakult account to Wieden & Kennedy.
Cdp-travissully's management team remains otherwise unchanged, with the creative director, Gill Sully, continuing to combine her role with that of managing director.
Ken Terasawa, the regional director of Dentsu in Europe, commented: "Simon displays exactly the sort of energy and drive that Dentsu needs in the leadership of its London operation."