He joins Robert Senior, the recently appointed worldwide chief executive, and Pablo Del Campo, the worldwide creative director, on the global leadership team for the two groups.
In the newly created role, Hackworthy will provide strategic leadership across the networks and work on key clients and new business.
Local chief strategy officers will continue to report to their own management team but there will be a dotted line to Hackworthy.
Hackworthy will continue to be based in London and retain strategic responsibility for Fallon London. He will maintain his current involvement with Fallon clients and also recruit a head of planning for the agency.