Concord chairman Alan Simmons said Mansell left last Friday, one day after making the announcement, and the parting was "entirely amicable".
Mansell was a shareholder in Concord's holding company Alban Communications, which Aegis acquired in a deal worth up to £8m in August last year.
Aegis plans to operate Concord, which has direct clients including Nestle and MasterFoods, separately to its main outdoor business Posterscope.
"He felt having done our deal with Aegis his job was done and after 33 years in outdoor it was as good a time as any to go," Simmons said. He added that Mansell was unlikely to be replaced.
Mansell was managing director of Concord from 1996 to 2004 before becoming chief executive.