Stewart, who was most recently the senior vice-president of global development at Publicis, will take charge of marketing for Emap's consumer brands across magazines, radio, online, mobile and TV.
He will report to Paul Keenan, the chief executive of ECM, and will work on integrating marketing as well as on new product and business development.
Stewart will take charge of Emap's £20 million adspend and its agency relationships. It currently appoints ad agencies to work on individual brands. These include Fallon on Grazia and Mother on Zoo. OMD handles media planning and buying for its magazines, while Starcom works across radio and TV.
Stewart's arrival follows a turbulent few months for Emap, during which it lost its chief executive, Tom Moloney, amid criticism of its digital strategy and falling circulations at key men's titles such as FHM, Zoo and Max Power.