The move comes just weeks after L'Oreal bought out the beauty division of Yves Saint Laurent -- YSL Beaute -- from the Gucci group in a $1.68bn (£1.12bn) deal.
ZenithOptimedia already handles the main L'Oreal business.
The YSL Beaute account has been with MPG since 2002, and will move to its new home at ZenithOptimedia in May 2009.
It includes a stable of perfume and cosmetics brands such as Opium, Touche Eclat fragrances, Elle and l'Homme.
L'Oreal acquired the business as part of a strategy to expand its presence in the luxury goods market, in which it already has Lancome, Biotherm, Shu Uemura, Kiehl's and Giorgio Armani perfumes.
The agreement hands L'Oreal the long-term worldwide licence for use of the YSL and Boucheron brands for perfume and cosmetics, and will also include the license for the beauty and fragrance divisions of Stella McCartney, Oscar de la Renta and Ermenegildo Zegna.