Henkel, the German FMCG manufacturer, called the review following its acquisition of the Bucks Chemicals business, which includes the Limelite detergent brand.
While the business internationally is still split equally between its three roster agencies - MEC, OMD and Carat -the UK becomes the first consolidated market.
Steve Harrison and Helmut Grosscurth, Henkel's heads of international media, oversaw the review.
They said: "We saw some very good presentations from the UK media agencies that, frankly, made discriminating between the agencies difficult and extended our decision process. After careful consideration, we are happy to continue our relationship with MEC, subject to finalising terms of business."
Toby Jenner, the client services director at MEC, said: "We are particularly pleased with this result as it firmly demonstrates that our Group M media negotiation strategy is already delivering benefits to our clients."
MEC was the incumbent on the largest part of the account, including the Schwarzkopf cosmetics brands.
Earlier in April, Schwarzkopf signed up as the sponsor of the repeats of Sex and the City on Channel 4, replacing the show's original sponsor, Bailey's, in a deal that was negotiated by MEC's specialist sponsorship division, MEC: Sponsorship.
The deal is intended to raise awareness of Schwarzkopf's Pro Styling range.