Until the end of 2013, Mark had been editor of the Liverpool Post in both its daily and weekly incarnations.
He then switched to the editor role at the Llandudno-based Daily Post at the beginning of 2014.
In a statement, Trinity Mirror said that Mark will leave on Friday June 26 to pursue “a number of new opportunities and projects.”
His career includes 14 years as regional correspondent for The Press Association, during which time he covered the Militant takeover of Liverpool city council, the disasters at Hillsborough and Heysel, the Warrington Bombing and the murder of James Bulger.
Mark went on to write a book on the Bulger case and is recognised nationally as one of the foremost authorities on the crime.
His awards include North West Young Journalist of the Year, North West News Reporter of the Year, and UKPG journalist of the year for his investigations into the Cream Nightclub financial irregularities and the lives in custody of the Bulger killers.
Mark began his career at Wirral Newspapers in 1975, before moving to the Post and Echo and then on to PA.
He left PA in 1997 to re-join the Echo as deputy news editor and moved through the ranks before succeeding Jane Wolstenholme as editor in 2007.
Said Mark: “’I’ve had such a rewarding 40 years in journalism. It’s been a fascinating journey and the camaraderie I have enjoyed has been every bit as important as the buzz and excitement of news coverage.
“I am very excited to be moving into a new phase in which I know the experience I have been lucky enough to gain working in the industry will be absolutely invaluable.
“I find it hard to believe it has been 40 years and while I know that the time is right for me to move on, I will greatly miss working with all my friends and colleagues at wonderful Trinity Mirror titles.
“The last few years have seen some of the most remarkable change in media history and it has been terrific to be at the heart of so much innovation and creativity.’’
Trinity Mirror said an announcement about Mark’s successor would be made in due course.