Ironside is being replaced by former MoS ad manager Stephen Miron, who is managing director of Associated New Ventures, which houses the Femail, This is London and UK Plus websites, contract publishing arm Cube, and Loot.
Miron, 38, joined Associated in his current role two years ago from The Independent, where he was commercial director.
In a statement Miron, said: "I spent 10 good years at the Mail on Sunday earlier in my career. It's a marvellous job and I am looking forward to the challenge."
Prior to that, he had spent 10 years at Associated working on the MoS, joining as a sales executive in 1988 and rising to sales manager and ad manager before heading to The Independent in 1998.
Ironside's departure comes as something of a shock to the industry. He was well liked and had been with Associated for 14 years, the last five of which have been as managing director of the Mail on Sunday.
He took over the job from Kevin Beatty in 1998 and his departure follows that of the paper's advertising director of three years, Sue Dear, who left earlier this year.
Dear's departure followed that of Mike Orlov, who resigned as advertising sales director of the Evening Standard a few weeks. Orlov was replaced by Alan Brydon, the former managing director of CIA Medianetworks.
The Ironside File
1974 FCB, media trainee
1978 Ted Bates, senior media manager
1979 Zetland Advertising, media manager
1980 TBWA, media manager
1984 Chetwynd Haddons, media director
1988 Time and Space, managing director
1989 Daily Mail, client advertising manager, becoming ad sales controller in 1992
1994 Daily Mail, ad director
1999 Mail on Sunday, managing director
The Miron File
1985 TV Times, sales assistant
1988 Mail on Sunday, sales executive
1991 Mail on Sunday, sales manager
1993 Mail on Sunday, advertising manager
1998 Independent, commercial director
2001 Associated New Ventures, managing director
2003 Mail on Sunday, managing director
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