Rick Hirst, the UK chief executive of Mcgarrybowen, is joining its sister media agency Carat in the same role.
In the new position, Hirst will seek to "define the next chapter" of growth for Carat.
Hirst will continue to report to Tracy De Groose, the UK chief executive of Dentsu Aegis Network.
De Groose said: "I think the lines are blurring between creative, digital and media agencies to help clients solve business problems."
Matthew Hook, who has led Carat as its managing director for more than four years, has been promoted to the new role of UK and Ireland chief strategy officer at Dentsu Aegis Network.
Matthew Landeman, the chief client officer, takes over as the managing director. He will work alongside Dan Hagen, the chief strategy officer, and Stuart Newman, the chief marketing officer.
Hirst joined Mcgarrybowen in 2013. Before that, he was the partner and client managing director at Engine. He has also worked at Fallon and Bartle Bogle Hegarty.
Dentsu Aegis Network is in the final stages of finding a replacement for Hirst at Mcgarrybowen.
De Groose added that the group is focusing on "hybrid leaders, collaboration and breaking down silos".