The Saatchi & Saatchi offices around the world will report to Billingsley. He has been tasked with leading growth and performance, and "transforming the capabilities of the business".
Billingsley replaces Chris Foster, who became the senior vice-president for global clients at Publicis Groupe in August.
Robert Senior, Saatchi & Saatchi’s worldwide chief executive, said: "Justin is a proven transformational leader for us – first in China, then in EMEA – and, in his new role, will focus on improvement in our performance globally.
"Justin and I have a great working partnership. It is as simple as this: we both stand in the doorway of the Saatchi & Saatchi network. From there, I tend to look outwards and Justin tends to look inwards. We need to do both in order to lift our game and achieve our purpose."
Billingsley took on his previous role in January. He joined Saatchi & Saatchi as the chief executive for Greater China in 2009 and, before that, was a brand director at Orange. Billingsley has also worked for Coca-Cola.