Karlsson, who will oversee the creative operations of McCann's New York and London offices, is one of the first high-profile moves made by Nick Brien since become CEO of McCann Worldgroup. Brien previously was CEO of Interpublic's Mediabrands unit, and was tapped to reengineer the venerable McCann organization.
Before co-founding Mother NY in 2003, Karlsson was a top creative at Fallon.
WPP, meanwhile, did not say explicitly how the acquisition of Taxi would change its existing organization, but announced that it would become part of its Y&R Brands division.
Founded in Montreal in 1992, Taxi currently has five offices in the U.S., Europe and Canada. WPP said Taxis management team would continue to run the organization, but that Founder and Partner Paul Lavoie would report to Peter Stringham, chairman-CEO of Young & Rubicam Brands.