73-year-old Sorrell had stated: "The current disruption we are experiencing is simply putting too much unnecessary pressure on the business".
The longest-serving CEO on the FTSE 100 blue chip index, Sorrell built WPP into one of Britain's biggest companies by three decades of relentless dealmaking.
In March WPP reported its weakest annual results since the financial crisis, with Sir Martin describing 2017 as "not a pretty year" for the company even though pre-tax profits were just over £2bn. Sorrell, a former investment banker who cut his teeth in advertising as the Saatchi brothers first finance chief, was a master deal maker at the fore of the global land grab. "The founder CEO has over 30 years' service with the company and is identified with the success of the group's strategy and a failure to plan for his succession could impact investor confidence in the company", it acknowledged.
We welcomed J. Walter Thompson, Ogilvy, Young & Rubicam, Grey, 24/7 Real Media, Taylor Nelson Sofres, among so many others. He is one of the most high profile, and best paid, executives in the country.
Sorrell sparred repeatedly with the long-time boss of French rival Publicis, Maurice Levy, and enjoyed pointing out to journalists the failings of his rivals.
Sorrell's departure comes at a hard time for the company as the industry struggles against tough competition from Google and Facebook. He earned about 200 million pounds ($284 million) over the last five years, largely due to a lucrative performance-related bonus package.
Sorrell's unique relationship with WPP allowed him to run it autocratically with an iron hand - leading to a "Sorrellcentricity" that critics say had made it his personal fiefdom.
The world's biggest ad company's share price was 3.5% down at 8am this morning but has ticked back up to -1.26% down from previous closing to 1,173p a share, despite speculation over the future of the company now Sorrell is leaving after 33 years.
However, de Groote predicted that if any of the major holding companies were to remain intact, it would be Publicis, because of its ongoing reorganization, recent leadership transition, and French business culture.
Sorrell's shoes are being filled by WPP Digital chief executive Mark Read and Andrew Scott, chief operating officer for Europe, on an interim basis as joint chief operating officers, but the multinational company could cast its net wider for a permanent successor.