The decision marks the end of the company's 16-year association with McCanns and one of the biggest of its kind outside London in recent years.
McCanns, which has been forced to repitch regularly for the business, finally lost out to CheethamBell JWT in a contest that also included another Manchester shop, Barrington Johnson Lorains, as well as two Leeds-based agencies, Brahm and PWLC.
Media planning and buying will remain with Universal McCann.
The review comes as Magnet, which has relied heavily on price-led advertising, faces increasing competition from the DIY "sheds".
Nick Booth, the company's head of brand marketing, cited CheethamBell JWT's "strategic and creative response to our brief across all disciplines" as the reason for the appointment.
"CheethamBell JWT has brought to us one big idea that builds on our current thinking but brings the campaign to life in a creative, fresh way," he said.
The account encompasses TV, press, in-store, digital, direct marketing, new literature and PR activity.
Magnet is part of Nobia Group, Europe's largest kitchen retailer. It enjoyed a 13% jump in kitchen sales last year while its operating profit climbed by 47% to £27.7m.
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