The three-year overhaul will cost WPP 300 million pounds ($380 million, 333 million euros), but seeks to save 275 million pounds annually by the end of 2021.
WPP will slash 3,500 jobs at its operations but will create 1,000 positions elsewhere, according to people familiar with the matter.
The move will largely impact administration roles as the firm seeks to merge 100 offices and shut 80 sites.
WPP employs more than 130,000 people worldwide, of which 14,000 are in Britain.
"The restructuring of our business will enable increased investment in creativity, technology and talent, enhancing our capabilities in the categories with the greatest potential for future growth," said chief executive Mark Read, who took the reins in September.
"As well as improving our offer and creating opportunities for clients, this investment will drive sustainable, profitable growth for our shareholders." The London-listed group has been in turmoil since Sorrell sensationally quit earlier this year amid allegations of personal misconduct.
WPP faced fresh woe in October after issuing a gloomy warning over weaker sales for 2018.