His decision to leave is said to have followed talks with Sir Martin Sorrell, the chief executive of WPP, who is preparing to ride to the rescue of the embattled Cordiant.
Hearn would this week only confirm his decision to quit but declined to comment further. Other senior 141 senior managers are understood to be leaving with him, fuelling speculation that they are destined for Publicis.
The Paris-based network was awarded £18 million worth of Allied Domecq business previously handled within 141, including brands such as Tia Lusso, Malibu and Ballantine's whisky, earlier this year. The switch was a major factor in plunging Cordiant into crisis.
A Cordiant staffer for 18 years, Hearn was key to the group's global British American Tobacco business. He had earlier declared his intention to do everything in his power to keep 141 intact. But with Allied Domecq gone and BAT yet to declare its intent, he is thought to have accepted that this would be impossible.
Meanwhile, sources at Cordiant say they are "completely bewildered" by the 10.75 per cent in the group built by Nahed Ojjeh, a wealthy Syrian chess promoter. Analysts suggest Ojjeh could be speculating in the hope of a rise in Cordiant's share price or could be part of an unidentified consortium attempting to beat WPP to take over Cordiant.