Sinnock will join M&C Saatchi before the end of the year and report to Lisa Thomas, the group chief executive. He will work with Richard Storey, the global chief strategy officer.
Before Bates CHI & Partners, Sinnock was at Ogilvy & Mather, where he was in charge of strategy and planning across Asia-Pacific.
Sinnock made his name at Fallon, where he helped to develop strategy on the Sony account, which included the "balls" campaign. He was also briefly a marketing director at Asda.
Thomas said: "We’ve felt for a while that we need strong leadership in the UK agency. We wanted someone who was young and hungry, and Mark’s CV certainly suggests he is that."