Myhren, 35, succeeds Tim Mellors, who was appointed Grey's chief creative officer in January with a brief to boost the network's global creative product.
Previously the executive creative director of Burnett in Detroit, Myhren oversaw all the agency's creative development. He was previously the creative director of TBWA\Chiat\Day in Los Angeles.
Steve Hardwick, the Grey New York president, said: "Tor's award-winning work will add a new dimension to Grey's creative product."