Luxmoore, who joined Mirror Group in September 2001, leaves two weeks after Mark Haysom, its managing director for national newspapers.
Sources close to Trinity Mirror said that Luxmoore had been asked to leave and that the company is looking for a successor. Phil Hall, the former News of the World editor who joined Trinity Mirror as the editorial development director in January, will oversee the role in the interim.
Sly Bailey, Trinity Mirror's chief executive, took personal charge of national newspapers following the departure of Haysom and is attempting to reverse fortunes at both the Daily Mirror and The People.
It has been a difficult year for the Daily Mirror and The People. The Daily Mirror's circulation recently dipped below two million for the first time and The People's declined 16 per cent in March.
While at Mirror Group, Luxmoore reviewed its creative account before deciding to leave it with M&C Saatchi.
Luxmoore was unavailable for comment.