The expanded role will see Hourston take on the strategic leadership of Grey's network of European offices while retaining his London responsibilities. He will also take a lead role in international new-business pitches.
The promotion is the latest in a number of senior appointments to the agency's EMEA region.
Last December, David Patton, the chief executive of the London office, was promoted to president and chief executive of Grey EMEA. Chris Hirst, the agency's managing director, replaced him.
A week later, Jon Williams, the executive creative director of Grey London, moved into the role of chief digital officer of EMEA and was replaced by the joint creative director Nils Leonard.
Hourston rejoined Grey, where he began his career, as its chief strategy officer in 2008. Before Grey, Hourston spent ten years at TBWA.
Since his appointment, the agency has won awards for the effectiveness of its work, including IPA Effectiveness Awards for its British Heart Foundation activity.
Patton said: "Neil's created a strong planning culture, attracted the best talent and I'm looking forward to working with him as he develops our strategic capabilities across the EMEA region."