Officially, Mr Montgomery is preparing his departure because he has lost the confidence of shareholders and non-executive directors. But behind that rather dry explanation lies a tale of intrigue, secret meetings and boardroom plotting.
It is a corporate battle which has set chief executive and chairman at one another's throats. It has also produced a classic clash of style and culture.
In one corner the dour, socially gauche Mr Montgomery, a working class loyalist from Bangor, Co Down. In the other, the wily and urbane Mirror Group chairman, Sir Victor Blank, a well-known investment banker.
To the victor, the spoils, is the convention in war. But in this case the postscript will almost certainly be the disappearance of the Mirror Group as an independent company. At least two predators are circling. With Mr Montgomery gone, Sir Victor will have removed the last impediment to the sale of Mirror Group to another newspaper publisher.
Mr Montgomery, a man not short of enemies in the media, was once described as someone who could lower the temperature of a room simply by entering it.
Within the industry he is known as something of an outsider, a cold, calculating figure with little time for social niceties who prospered by a combination of brains, hard work and, when necessary, ingratiation.
But his business acumen has not saved him this time. Mr Montgomery's fate was sealed early yesterday when he met Mirror Group's biggest shareholder, the giant pension fund manager Phillips & Drew, run by Tony Dye. The Mirror boss was told that he had two choices: either to go voluntarily with his dignity intact or to be forced out ignominiously by a vote of the board or at a special shareholders' meeting.
Last night, Mr Montgomery was still clinging to power and claiming the support of his fellow executives and at least one non-executive. But barring a miracle of Biblical proportions, his departure will be confirmed after a board meeting today.
Mr Montgomery has no shortage of detractors in national newspaper journalism ready to share a cruel story about "Monty". David Banks, a former editor of The Mirror, used to call him the Cabin Boy because of the way he "sucked up" to people.
Aside from Robert Maxwell, he is arguably the most unpopular newspaperman ever to walk Fleet Street. His talent was for cutting costs, a speciality that made him a hate figure among those whose jobs he eliminated and those whose journalism he undermined.
Yet in the eyes of the City, Mr Montgomery is reckoned to have done a reasonable job and in the early days of his stewardship he had what passes for a fan club in the Square Mile. Since rescuing Mirror Group from the ruins of the Maxwell era he has been steady progress, with one or two exceptions such as its foray into L!ve TV.
When he took over the share price was languishing at less than 60p. It is now above 200p and after a long period of stagnation the flagship title, The Mirror, is now clawing back ground in the circulation battle among the red tops.
But ultimately, cost-cutting can only take a business so far. For an encore, Mirror Group's big City shareholders have decided they want someone who can take the business forward and earn them a better return either by selling out or merging with a rival publisher.
Mr Montgomery, apparently, is not that man. He does not feature in the future plans of either of the two bidders who have so far declared their hands - the regional newspaper group Trinity and Regional Independent Media, which is chaired by the former Conservative party chairman, Sir Norman Fowler, and publishes the Yorkshire Post among its titles.
After a cat-and-mouse game lasting six months, Sir Victor has concluded that Mr Montgomery has never been interested in any deal that would undermine his own power base. As one adviser to the Mirror chairman put it: "The fact is Montgomery would be an impediment to any deal ... because he has decided that saving his own skin is more important than serving the interests of shareholders."
As an ex-editor of both the News of the World and Today, Mr Montgomery has not been afraid of taking the Fleet Street battle to his enemies. The Montgomery camp has had three spin doctors working for it including David Burnside, a former head of PR at British Airways at the height of the dirty tricks saga
Lo and behold, last weekend's press was full of anti-Blank stories accusing him of making secret contact with potential bidders without the knowledge of the rest of the board and meeting Mirror shareholders without the company's advisers being present - something which breaches corporate governance principles.
Mr Montgomery will be well-rewarded on his departure. He is on a two year contract and earns more than pounds 500,000 a year. In addition he has 400,000 share options left having already made pounds 650,000 from cashing in other options.
Nor does his career path suggest that the media world has heard the last of David Montgomery.
Born in 1948 into a loyalist Presbyterian family, his first foray into journalism came at Queens University where he edited the student newspaper, The Gown, an editorship that involved writing disapproving stories of wasteful students spending their grants on booze.
After graduating, he moved to Manchester on the Daily Mirror's training scheme where he was remembered as a hard-working journalist who showed more of an interest in production than writing - an early sign of his determination to be an editor.
According to Chris Horrie, author of L?ve TV, an account of Montgomery's downmarket foray into cable television,Derek Jameson, the head of the Manchester office, remembers repeatedly telling him to "piss off" after being pestered for extra work every night.
In 1980, Montgomery moved to London and later joined Nick Lloyd at The Sun. Lloyd was to become editor of the News of the World with Montgomery as his deputy but within three years Montgomery took over. In 1987, Rupert Murdoch bought Today and installed Montgomery as editor. He was quick to identify a new market - aspirational wannabe yuppies who wore power suits, dreamed of driving Porsches but who claimed to have an interest in the environment. Montgomery called his new constituency the "Green Greedy People".
After an unsuccessful management buyout attempt he found himself at the door of Lord Hollick of the MAI financial services group. Hollick recommended he be installed as chief executive of Mirror Group, an appointment approved by one vote.
L!ve TV was perhaps his lowest point, appointing the former Sun editor Kelvin MacKenzie to churn out programmes featuring topless darts, the news bunny and a Norwegian weather forecaster in a bikini.
His passing is not likely to be mourned by journalists at The Mirror. They, after all, were the ones who nicknamed him "Rommel" - because at least Monty was on our side.
Sir Victor Blank, City deal maker and former chairman of investment bank Charterhouse
Brought in as Mirror Group chairman to find a buyer or merger partner. Concluded that was impossible with a chief executive more interested in "saving his own skin than representing shareholder interests".
Sir Norman Fowler, former Conservative cabinet minister and chairman of Regional Independent Media.
Made 200p-a-share bid for Mirror Group. Questions about his contacts with the Mirror chairman Sir Victor Blank and whether he would be suitable to run a Labour-supporting paper.
Tony Dye, head of Phillips & Drew, Mirror Group's largest shareholder.
Threatened to call an extraordinary meeting of shareholders to oust Mr Montgomery if he did not agree to go voluntarily. He backs an all-share offer for Mirror from the regional newspaper group Trinity.Reuse content