McLennan, who spent the past two years as the executive creative director at Lowe Bull, succeeds Mick Petherick, who has become the creative chairman of JWT Johannesburg.
Peter Sham, the chief executive of JWT South Africa, said: "JWT is leveraging its creative standards on a global basis. The appointment of Rob builds on that of Conn Bertish as creative director."
McLennan started his career at CDP before forming his own design shop, Graphic Link, in 1986. This merged with Column Advertising in 1989, which was then purchased by Lintas in 1999.