Mark Sinnock, the chief strategy officer and a partner at Fallon, has quit to become the marketing director of Asda.
Sinnock will take responsibility for developing the marketing and customer strategy across the breadth of Asda's marketing function, working alongside Katherine Paterson, Asda's marketing director for communications.
The appointment follows the promotion of the former marketing director Rick Bendel, who took on the role of chief marketing director at the supermarket in February.
Sinnock's appointment is the second to Asda's marketing department in a week. The McCain Foods marketing director, Simon Eyles, was appointed as the head of brand marketing on 6 March, reporting to Paterson.
Both Sinnock and Paterson will report to Bendel.
Sinnock, whose career has already included stints client-side, at O2 and NTL, joined Fallon as a planning director in 2004. He took up the strategic lead on the Asda account when the agency snared it from Publicis in 2007.
Fallon's appointment to the £70 million account came just three months after Bendel, the former Publicis chief operating officer, defected to the supermarket from the 17-year incumbent.
As a result of Sinnock's departure, Fallon is promoting its head of planning, Rachel Barrie, to the role of chief strategy officer.
Barrie, who heads the Cadbury and BBC accounts, joined Fallon as a senior planner from Partners BDDH five years ago.
Gareth Goodall, a senior planner at Fallon, will step up to fill Barrie's role.
Bendel said: "Mark has a unique talent for turning information into insight, making the complex simple and he loves Asda - it's an irresistible combination."
Sinnock said: "I've had five memorable years as a partner at Fallon but this is a rare and incredible opportunity and one that I intend to grasp with both hands."