He will initially focus on Havas Worldwide, Havas Helia and Havas Work Club.
In the newly created role, Sinnock will report to Chris Hirst, the European and UK group chief executive at Havas Creative Group.
Sinnock spent two years at M&C Saatchi’s London office. Before that, he was the chief strategy officer at Bates CHI & Partners in Singapore between 2012 and 2013.
In 2010, Sinnock became the president for planning and strategy across Asia-Pacific at Ogilvy & Mather.
He has also spent time at Fallon in a strategy position and at Asda as its marketing director.
Havas is undergoing changes before it moves to the Havas Village in King’s Cross in 2017.
In September, Hirst joined the company from Grey London. In the same month, Paul Ward, the former chief creative operations officer at Naked Communications, became the group head of operations and innovation.
Hirst said: "Mark is a stellar talent with an enviable breadth of experience, from hotshop to FMCG marketing director.
"He’s going to make us smarter and help us build world-class solutions around the needs of our clients."