The appointment of Watson, who spent ten years at the Mirror Group's bitter rival, follows recent management changes since the arrival of the Trinity Mirror chief executive, Sly Bailey.
Senior managers including Mark Haysom, the managing director of national newspapers, and the marketing director, Alisdair Luxmoore, have left Trinity Mirror since Bailey's arrival.
Watson, 35, is currently the managing director of Celador International.
Before this he held several roles in News Corporation, including that of marketing director at The Sun and News of the World publisher, News Group.
Among other things, Trinity Mirror will hope Watson's new role, which sees him take direct responsibility for advertising, circulation and marketing, will help revive flagging circulations of its key titles Daily Mirror, Sunday Mirror and The People. His responsibility for Mirror Group's marketing and advertising strategy will include managing its relationship with its creative agency, M&C Saatchi. The agency was reappointed by Mirror Group last year following a pitch organised by Luxmoore.
The editors of Mirror Group's titles will continue to report directly to Bailey.
She said: "Ellis has enormous commercial experience in media and, in particular, national newspapers. He will bring a renewed commercial focus and vigour to the operation."