Carat has held the account for more than a decade but General Motors, the parent company of Vauxhall and Opel, sold the division to PSA Group, the owner of Peugeot and Citroen, earlier this year.
MediaCom already held the media account for PSA Group's existing business, after winning a competitive review for the estimated €875m (£773m) business in June 2017, beating the previous incumbents OMD and Havas which shared most of the business previously.
The media-buying for Vauxhall-Opel was not included in that review as PSA Group was still completing the acquisiton.
PSA Group went on to conduct a closed process this autumn to decide the Vauxhall/Opel media account and awarded it to MediaCom.
Vauxhall/Opel spends in the region of £35m on media in the UK, depending on estimates.
Peter Küspert, managing director sales and marketing, Opel Automobile, said it wanted to "significantly increase the efficiency of our marketing spend" and it will "achieve great synergies" by merging its ad budget with PSA Group.
Küspert also thanked Carat for its "excellent support".
Stephen Allan, the global chairman and chief executive of MediaCom, said: "MediaCom is delighted to have earned the opportunity to apply its unique expertise and scale to Opel Vauxhall in the years to come.
"These two brands, alongside Peugeot, Citroen and DS, are some of the most revered and historic in the automotive industry."
MediaCom has been on a mission to win automotive business after losing the £2 billion Volkswagen Group global account last year.
Vauxhall/Opel declined to comment on the size of its media spend.