Sir Martin Sorrell has seen off competition from WPP, the firm he ran for three decades, with the first acquisition for his new rival venture.
S4 Capital is buying Dutch digital production company MediaMonks for about €300m (£266m).
Earlier this month, WPP threatened to take away share awards worth about £20m from its former chief executive over his rival bid for MediaMonks.
Sir Martin resigned as WPP chief executive in April.
He left the world’s biggest advertising firm amid claims of misconduct, which he has denied.
The findings of an internal investigation into the allegations have not been disclosed because WPP said doing so would breach data protection rules.
Soon after leaving Sir Martin set up his new advertising venture, which he has pledged to build into a “multinational communication services business”.
Earlier this month, a source close to Sir Martin described relations between WPP and its former boss as “obviously not very good”.
The source said WPP’s threats to take away his share awards would not deter Sir Martin: “This guy is worth £400m to £500m. He is not going to allow £20m to stand in the way of what he is trying to do.”
Sir Martin’s decision to start a new venture has echoes of his reverse takeover of Wire and Plastics Products in 1985.
The maker of wire shopping baskets was turned into WPP, and over 30 years became the world’s biggest advertising company with revenues of over £15bn.