Willis, who was ad manager for The Observer, is leaving to become head of trading at the Mail on Sunday, where he will report to Simon Davies, the new advertising director at the mid-market tabloid.
Pelekanou takes over the joint role on July 8. His new job will coincide with a restructuring of the department to focus more clearly on the company's brands.
Pelekanou began his career with VNU Publications on Computing magazine before moving to Quantum Publishing. He joined Guardian Newspapers as food and drink executive in 1992.
Stuart Taylor, advertisement director for Guardian Newspapers, said: "Chris Willis' departure after nine years of outstanding contribution to the company is sad news and we wish him the very best of luck at Associated."
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