The businessman, who has built up the company over 32 years, resigned over the weekend following allegations of personal misconduct through the misuse of company assets, confirming a decision first revealed by Sky News .
Sir Martin, 73, will be treated as having retired from the company, meaning he will be entitled to a maximum 1.65 million shares under long-term award plans dependent on WPP's performance.
At the current share price, they are worth just under £20m.
Sir Martin already owns shares in WPP worth more than £200m, while his annual remuneration packages - he earned £70m two years ago - have made him a target for critics of lavish boardroom pay.
"As I look ahead, I see that the current disruption we are experiencing is simply putting too much unnecessary pressure on the business," he said.
"That is why I have decided that in your interest, in the interest of our clients, in the interest of all share owners, both big and small, and in the interest of all our other stakeholders, it is best for me to step aside.
"As a founder, I can say that WPP is not just a matter of life or death, it was, is and will be more important than that. Good fortune and Godspeed to all of you … now Back to the Future (Frankfurt: 923414 - news) ."
In a statement, WPP said: "The previously announced investigation into an allegation of misconduct against Sir Martin has concluded. The allegation did not involve amounts that are material."
Sir Martin, who acquired the small Kent-based firm in 1985 and turned it into the world's largest marketing services group, will be treated as having retired, the company confirmed.
Chairman Roberto Quarta will become executive chairman until a new chief executive has been appointed.
Sir Martin denied any wrongdoing after the allegations surfaced earlier this month, but said he understood the company had to investigate it.
He previously worked at Saatchi & Saatchi, and was knighted in the Queen's New Year honours list in 2000.
He said: "In the coming period, I will be available to the board and any of you, should you want help with anything, anywhere."
Sir Martin has established himself as one of relatively few British business leaders who are recognised around the world.
He has cultivated a global reputation as a commentator on economic and political matters, forging relationships with government ministers, financiers and corporate bosses around the world.
However, his departure will leave the company he built virtually from scratch facing profound questions about its future direction.
The marketing (Frankfurt: 2MG.F - news) services industries are being buffeted by accelerating trends such as zero-based budgeting, which has led to many consumer-facing companies abandoning increases in marketing spending.
WPP has seen its shares fall by nearly a third during the last 12 months, although it still has a market capitalisation of more than £15bn.