The company is now hunting for a new CEO to replace Sorrell who has been in the role for 33 years.
In the statement issued about Sorrell's resignation, WPP says the investigation into the allegations has concluded and "did not involve amounts that are material." The statement did not address whether those amounts and the allegations of misconduct were separate matters.
Sorrell said in a statement that it's in the company's "best interest" he steps down.
"Obviously I am sad to leave WPP after 33 years. It has been a passion, focus and source of energy for so long," he said.
"I leave the company in very good hands, as the board knows. Mark and Andrew and the management team at all levels have the knowledge and abilities to take WPP to even greater heights and capitalise on the geographic and functional opportunities. I will particularly miss the daily interactions with everyone across the world and want to thank them and their families for all they have done, and will do, for WPP."
WPP said in accordance to his at-will employment agreement, Sorrell will be treated as having retired on leaving WPP.
"Sir Martin has been the driving force behind the expansion of WPP to create the global leader in marketing services. 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," Quanta said in a statement.
"On behalf of the Board I would like to recognise these achievements and thank Sir Martin for his commitment to the business over more than three decades."
Prior to the recent investigation, Sorrell took a significant pay cut in the wake of WPP's steepest decline in stock value since 1999.
In addition to the official statement, Sorrell also emailed each of WPP's global employees, addressing the succession plan and adding that the business is under "unnecessary pressure".
He finished the email asying: "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."
The news is likely to impact the share price of WPP as well as have wider implications for the advertising industry, including WPP AUNZ.
WPP AUNZ has been under its own pressure in the first quarter of this year, which was addressed last week by AUNZ CEO Mike Connaghan. Read more here.
Martin Sorrell makes shock departure from WPP
By Lindsay Bennett | 15 April 2018
Martin Sorrell has made a shock departure from WPP amidst allegations of personal misconduct.