The deal for Vauxhall and Opel (the non-UK car brand) means PSA Group would become Europe’s second largest carmaker behind Volkswagen.
The deal includes PSA buying GM Europe's financial operations for €900m in a joint deal with bank BNP Paribas. PSA said it expected to make savings of £1.47bn per year by 2026.
The two firms announced they had been in talks over the deal since 14 February.
Carat has handled global media for General Motors since 2012, while OMD UK handles media planning and buying for PSA in the UK.
A separate Vauxhall review is underway for its marketing services account, which covers CRM, point of sale, print and retail marketing. McCann Group (including MRM Meteorite), Geometry Global, Oliver and Graymatter are pitching for the business, with the decision due in May.
Carlos Tavares, chairman of PSA's managing board, said: "We are confident that the Opel/Vauxhall turnaround will significantly accelerate with our support, while respecting the commitments made by GM to the Opel/Vauxhall employees."