McLennan, the former chairman and chief executive of Young & Rubicam Brands in Australia, will take over later this year. Fudge will remain as the chairman and chief executive of Y&R Brands, a unit that includes Wunderman and Burson-Marsteller.
McLennan's appointment follows an 18-month search after Fudge announced her plans to relinquish her leadership of the advertising network in 2005.
McLennan's wide-ranging remit will include kick-starting the network, working on Y&R's digital offering, prioritising new business and focusing on creative.
In the past two years, the network has seen the loss of its Jaguar, Sony Electronics and Burger King accounts. In January, the network also laid off 10 per cent of staff in its New York office because of account losses.
McLennan joined Y&R in 2002, after 17 years at George Patterson Bates.
In 2005, he managed Y&R's merger with Patterson Bates and assumed direct control of the agency. He also held responsibility for the enlarged group, which included the operations of Patterson's parent, The Communications Group.
McLennan said: "It's a great privilege to be part of the Y&R global team and I look forward to working with our people and our clients to lead our business into the future."
Fudge added: "Hamish has been a rising star at the agency. He has an outstanding track record in the business and I am delighted to have him as my partner at Y&R."