Davies joins as the chief operating officer of McCann-Erickson World Group EMEA, taking over the position vacated by Brian Watson at the end of last year.
Davies joins from the agency's Dublin office, where he was managing director.
He will be replaced by McCann-Erickson Ireland's client services director, Orlath Blaney.
The move follows the departure of McCann-Erickson UK's chairman, Tamara Ingram, who departed last week having rejected proposed changes to her role.
Ingram's departure marks the start of a restructuring process that will take place over the coming months. Langdon is expected to promote an internal candidate to replace her rather than bringing in a big name from outside.
The deputy chief executive, Chris Hunton, the managing director, Nick Wright, and the European client services director, Stephen Whyte, are the most likely candidates for the position.
Langdon has taken temporary charge of the UK agency and once a new chief executive has been appointed he is expected to have greater involvement in the UK office's day-to-day running than he did during Ingram's tenure at the agency.