Richard Beaven, chief executive of Initiative, said: "We are thrilled to have been selected as media and marketing partner with Hyundai and Kia, two of the fastest-growing players in the automotive category.
"Like us and our existing clients, Hyundai and Kia believe in the power of media to transform marketing results, and we are keen to work with such like-minded individuals during this important and challenging time for the industry."
World Marketing Group, the media management group for Hyundai and Kia, put the account up for review in October last year and hired Roth Associates to handle the process.
Sarah Fay, chief executive of Carat, said the account loss was "sad news" for Carat, but it gives the agency the chance to pitch for another car account.
Fay said: "This has been an important, successful piece of business... We still see a future in the automotive business."