He will replace Colin Clark, who has announced he will retire at the end of the year. Rayner has worked alongside Clark as the deputy managing director for the past year.
Rayner will run the agency and report directly to Jerry Hill, the joint chief operating officer of Initiative EMEA.
Rayner said: "I am delighted to be taking over from Colin. The business under his stewardship has made great strides, especially securing eight new clients in the past 12 months. He will be sorely missed, not only by our agency, but also by our clients and all people connected to our industry."