"Finding someone who is a charisma machine with chief executives in the same way Martin is will be tough", said Claire Enders, the founder of Enders Analysis.
But beyond the investigation that leaked this month, there were other signs Sorrell was losing his magic touch.
The firm, which has offices in Mayfair, New York, Mumbai, Hong Kong, Singapore, Seoul and Shanghai, said an investigation into an allegation of misconduct has concluded.
The resignation brings an abrupt end to the 33-year reign of Sorrell, during which he transformed a small shopping basket manufacturer into a global advertising powerhouse. It made no further comment but repeated a previous statement that the allegation did not involve amounts that were material to the company.
"Obviously I am sad to leave WPP after 33 years", Sorrell says in a statement. "Good fortune and Godspeed to all of you".
Roberto Quarta, WPP's chairman, has been appointed executive chairman until the appointment of a new chief executive.
"During this time, the company has been successful because it has valued and nurtured outstanding talent at every level - within and well beyond our leadership teams".
Mark Read, chief executive of Wunderman and WPP Digital, and Andrew Scott, WPP corporate development director and chief operating officer for Europe, have been appointed as joint chief operating officers of WPP.
Sorrell's 1985 investment in Wire & Plastic Products and string of acquisitions of advertising companies effectively founded WPP, making him an executive seen by some as irreplaceable for his direct access to clients and active hand on a sprawling network of agencies. "Most of my wealth, if not all of it, is and has been for the last 31 years tied up in the success of WPP". His share awards will be pro-rated in line with the plan rules and will vest over the next five years, to the extent that performance targets are met. Sorrell is one of the largest shareholders of WPP, with a 1.46% stake, according to data compiled by Bloomberg.