He is believed to have left in order to head up a bid for the commercial radio station Talk Radio. He is being advised by Apax Partners, the financiers behind Chris Evans' successful pounds 85m bid for Virgin Radio.
Mr MacKenzie, 52, was brought in by Mirror Group chief executive, David Montgomery, to work on the cable channel Live TV! but was promoted to run the group's newspaper operations in January. Mr MacKenzie was supposed to oversee an investment in journalism at the Mirror to give the newspaper a slightly more serious look.
Talk Radio, the national station, has been on the market since its major shareholder the Luxembourg media group CLT-UFA announced that it would sell all its UK radio stations earlier this year.
A source at Talk Radio said yesterday that there had been a number of bids for the station but that no decision had yet been made. Bidders are believed to include the US broadcaster Jancor Communications and the regional rolling news station, London News Radio.
Talk Radio has been struggling to make money since its launch in 1995 because of the size of its cash bid to win its national speech radio licence.
There has been speculation about Mr MacKenzie's future since it was revealed two weeks ago that the German publishing giant Axel Springer is considering a takeover bid for the media group.
Axel Springer, publisher of Germany's biggest selling tabloid, Bild, is run by Gus Fischer, a former boss of Mr MacKenzie during his time at the Sun. The two are believed to have had strained relations in the past. City advisers said yesterday they would mark down the value of Mirror Group without Mr MacKenzie as part of the management.
Mr MacKenzie is known to have requested the sale documents on Talk Radio months ago and is now thought to be part of another consortium. "He has been chillingly happy recently," said a Mirror executive yesterday. "He has always said he wanted to get back into broadcasting."
While at the Sun, the former South East London Mercury reporter became infamous for his brand of xenophobic and homophobic journalism. His style was best illustrated by his headline used after the loss of 368 lives in the sinking of the Belgrano during the Falklands War: Gotcha!
He attracted equal opprobrium and cost the Sun 200,000 sales, worth an estimated pounds 10m, in Merseyside after the Hillsborough disaster when it ran a story headlined "The Truth" which claimed: "Some fans picked pockets of victims. Some fans urinated on the brave cops. Some fans beat up PC giving kiss of life."