He joins the managing director, James Murphy, and the head of planning, David Golding, to complete the agency's succession management following the end of the founders' earn-out in October. Priest will work under the executive creative director, Mark Roalfe.
Roalfe said: "We looked at a lot of the creative talent in London and Ben is part of a new breed of creative directors. He has a great creative reputation and is naturally charismatic. He has shown he is somebody who is ready to make the next leap and take on the 'big job'.
"We now have the right structure in place to drive the agency forward and develop the business even more."
Copywriter Priest said: "It was simply an irresistible offer. The agency is already in fantastic shape. My aim is to keep it great where it is and help it to improve where it needs to."
Priest, 35, joined TBWA in 1999 as a creative group head with responsibility for PlayStation, Holsten Pils and Thomas Cook.
It was while at TBWA that he built up a successful partnership with the art director Brian Campbell. The duo, who made a name for themselves at Lowe Howard-Spink, were behind the much-vaunted "It's time to leave the country" campaign for Thomas Cook, "3G" for Hutchinson, and PlayStation 2's "Bambi".
They also created the "It's the daddy" series of ads for Holsten starring the British film hard man Ray Winstone.
Following these achievements, in 2001 Priest was promoted to managing partner at TBWA.
He launched his advertising career in 1991 as a copywriter at Ogilvy & Mather.
In 1993, he moved to Collett Dickenson Pearce where he was awarded a silver at the 1995 Campaign Poster Awards for his work on the "mannequin" ads for the cigar brand Hamlet.
Between 1995 and 1999, he was a copywriter for Lowe. During his time at the agency he picked up three D&AD silver Pencils for his work on Olympus cameras, a silver at the 1997 Campaign Poster Awards for Reebok and a silver at the ceremony the following year for Vauxhall.
Jim Kelly the RKCR/Y&R chief executive, said: "It was immediately apparent when we met him that Ben was a natural leader. His hiring is the last piece of the jigsaw in creating management succession at RKCR/Y&R."