De Groose, who was promoted to chief executive of Dentsu Aegis Network UK in May from her previous role of chief executive of Carat, will also take on additional management responsibility for Ireland.
Horler, who is currently the chief executive of Northern Europe at Dentsu Aegis Network, will relocate from London to New York in April.
He will continue to report to Nigel Morris, the chief executive of Dentsu Aegis Network, Americas and EMEA.
Horler joined the business as the managing partner of Carat Interactive in London 2000. He went on to found the digital media agency Diffiniti, which became part of the iProspect network in 2009.
After becoming the managing director of Carat UK in 2008, Horler was promoted to chief executive for Aegis Media UK in 2011. Horler assumed his current role as the chief executive of Dentsu Aegis Network Northern Europe in 2013.
Horler will retain his responsibility for the ongoing global development of Dentsu Aegis’ iProspect and Data2Decisions brands.
The Northern Europe chief executive will not be replaced directly. Other changes include Thierry Jadot, the chief executive of Dentsu Aegis Network France, taking on responsibility for the oversight of the network in the Netherlands.
A version of this story was first posted on Campaignlive.com.