MediaCom will handle media planning and buying for the body, as well as fulfilling a strategic consultancy role. The agency has previously worked on a project basis for Thinkbox, handling media planning and buying.
The appointment follows that of The Red Brick Road as Thinkbox's first creative agency in November last year.
MediaCom will help deliver strategy and advise on the wider TV market, general trends and insights and driving best practice. The account team will be headed by MediaCom's managing partner, Nigel Robinson, and the planning manager, Stephen Ward.
Lindsey Clay, the marketing director at Thinkbox, said: "MediaCom's wealth of insight, expertise and ambition - together with its clear understanding and appreciation of TV - makes it the perfect partner."
Robinson said: "We feel extremely privileged that Thinkbox has selected MediaCom to help it promote TV to the wider media industry."