Mawer became the commercial leader for Richard Desmond's media group behind the Express and Star newspapers in August.
He had been promoted from the role of digital director, where he had been responsible for programmatic sales, expanding audiences, partnerships and optimisation.
It is unknown if Mawer will be replaced. Jane Putley, who has been at Express Newspapers for 20 years, is currently the group trading director with responsibility for all newspaper ad sales.
In October, Northern & Shell revealed it had returned to profit in 2013 when falls in print revenues had been offset by record revenues of £359.7m for the group's TV operation, Channel 5.
However, with N&S having since sold Channel 5 to Viacom for £463.3m, its remaining printing and publishing business is hoping to stabilise after revenues fell 11%, from £286m to £257.3m, in the last year.
No one from Northern & Shell was available for comment at time of publication.
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