The appointment ends a near six- month pitch process that has seen Havas' MPG, WPP's Mediaedge:cia and Publicis Groupe's recently launched VivaKi operation knocked out along the way.
The Renault Nissan Purchasing Organisation (RNPO) has confirmed the new consolidated automotive account will be worth €800m (£642m) and includes European media buying across 24 countries for Nissan and 30 countries for Renault.
Nissan's media account was worth in excess of £150m in 2007, which includes UK billings of £28m, according to Nielsen Media Research, handled by Omnicom's Manning Gottlieb OMD.
Carat, meanwhile, has been incumbent for Renault's £400m pan-European media business for the past eight years, including £47m worth of ad spend in the UK, according to Nielsen.
The RNPO was created in April 2001 as the first joint subsidiary of the Renault-Nissan Alliance. However, it was not until earlier this year that its remit was extended to include the marketing and advertising services across the two global companies.
Both Renault and Nissan's three- year contracts were up for renewal in 2009, leading the Renault-Nissan Alliance to look at consolidating the business to create one of the biggest media accounts in the world.
The group's first-string agency network OMD already handles Nissan's £400m media account in the US, where the Renault brand is not marketed.