McCann notified its employees about the loss this morning in an internal memo.
The move ends a seven-year relationship between the diet marketer and the Interpublic Group of Cos. shop. It also comes as Weight Watchers is steered by a new leadership team. Lesya Lysyj, the former chief marketing officer at Heineken USA, took over as president of Weight Watchers North America late last year. The company also recently parted ways with senior VP-marketing Cheryl Callan.
The marketer and Wieden did not respond to requests for comment on the prospect of hiring Wieden. Ms. Lysyj is familiar with Wieden from her time at Heineken, when she named the agency the lead shop for brand Heineken in 2011.
"There have been significant recent changes at Weight Watchers, including a new CEO, a new President, and a search for a new North American CMO," said Chris Macdonald, president of McCann, New York, in the internal memo, which noted that the relationship would end in June. "It makes sense to part ways."
Weight Watchers spent $149.8 million on measured media in 2013, a figure that includes branded food products, according to Kantar Media. The last significant campaign McCann did for Weight Watchers starred Jessica Simpson.
The agency change comes as traditional diet marketers like Weight Watchers struggle to adapt to changing consumer behavior, as more dieters look toward competing online solutions, including free weight-loss apps.
"Our offerings have become less appealing in this changing market, largely because our innovation approach and our approach to the consumer in general have not been sufficiently market-driven," Weight Watchers International CEO James Chambers said on an earnings call in February. He expressed confidence in the marketer's new diet program, called Simple Start, but said "the marketing introducing it has been less effective."
"The net of this is that we have not been able to combat the increased pressure on consumer consideration from free apps and activity monitors," he said.
In the McCann memo, Mr. Macdonald said: "We can be very proud of the strategically smart and creatively superb work we've done over the past seven years spanning broadcast and online, including the stunning Jennifer Hudson campaign, the break-through outreach to men starring the inimitable Charles Barkley, and more recently the relatable and very human Jessica Simpson work."