The award is based on the total number of points won by WPP agencies from more than 50 countries throughout the week. Omnicom and Publicis Groupe finished in second and third place, respectively.
WPP's Ogilvy & Mather won network of the year for the fourth time running.
The winning work included Grey London’s "life paint" for Volvo, which won two Grands Prix in the Design and Promotion and Activation categories.
Other winners were Ogilvy Beijing’s "great Chinese names for Great Britain" for Visit Britain and AKQA London’s "house of Mamba" for Nike.
RGA/New York was crowned Agency of the Year by the Cannes Lions judges. The Interpublic shop received the award as the festival concluded last week, with Grey New York coming second and Ogilvy Brasil, Sao Paulo, placed third.
Adam & Eve/DDB took home the trophy for the UK last year, the only British winner of the award since 2002.
Sir Martin Sorrell, the founder and chief executive at WPP, said: "We are delighted to have made it a quintuplet (or should I say quintuplé?) of wins in Cannes.
"The holding company award is special because it is a shared achievement, reflecting the hard work, passion and commitment of every member of the WPP family. A big thank you to our agencies and all the hugely talented and creative people within them.
"However, this week of celebration has been overshadowed by loss and sadness for many people after the untimely deaths of two colleagues and friends during the festival.
"On behalf of WPP I would like to extend our deepest sympathy to everyone affected by these tragic events."
Sorrell was referring to the deaths of Google executive James Howard, and a journalist during the festival.