Williams and Fergusson step into their new roles this month.
UK creative agency leader moves up to global role with management committee position.
He replaces Nicky Lim, who leaves after a six-year stint at the network.
He takes on newly created post in addition to his responsibilities as chairman of Publicis Emil and Publicis One Touch.
First quarter revenue will be published April 23rd before the market opens.
'The last 12 months have been a tough fight for transformation, but without a doubt a necessary one,' holding company chief said.
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