Farr, who will report to the Group M chief strategy officer, Nick Emery, will work closely with MindShare's econometric company as well as the ATG and Consumer Insights teams.
Farr's work on consumer purchase behaviour was one of the central planks of Group M's recent channel planning research, Connections.
Before the move, Farr worked at Millward Brown's brand valuation and strategy consulting arm, MBOptimor.
Farr oversaw the development of the Brandz Top 100 ranking of the world's most powerful brands and the new league table of global brand financial value.
Emery said: "Farr has been the architect of some of the industry's leading research approaches. It's great to be able to bring in someone of his talent. His work on brand evaluation leads the market."
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