Farish, who was appointed to the Haymarket Media Group board in 2003, departs after more than 20 years at Haymarket, which owns Mediaweek.co.uk, amid a review of the company led by Haymarket Media Group chief executive Kevin Costello.
Haymarket Business Media chairman and managing director Martin Durham stepped down from his position at the helm of HBM in December, prior to Costello's appointment in January.
Durham's departure followed the decision by Michael Heseltine, the founder and executive chairman Haymarket Media Group, to hand over the reigns to his son Rupert and take on the Haymarket Group chairman role.
In a statement to staff this morning, Lord Heseltine thanked Farish for his "invaluable" contribution to the company.
Heseltine said: "He has ensured that our brands have been dominant in their marketplaces and more recently demonstrated effective new ways for delivering content for the future, through his strategy reviews."
Farish joined Haymarket as a trainee journalist in 1985, rising through the ranks to become managing director across six B2B sectors, responsible for brands such as Management Today, PRWeek, ENDS, Planning, Nursery World, and Third Sector.
Farish said: "Although I have hugely enjoyed my time at Haymarket – and especially working with so many talented colleagues – there are new opportunities now emerging that will transform the media sector in the years ahead, and I am excited by the idea of pursuing them."