Effective from 2 October, she replaces the former chief executive, Garry Lace, who quit the agency in May.
The appointment, which was made by Steve Gatfield, the chief executive of Lowe Worldwide, and its chairman, Tony Wright, is the final piece in a new management team assembled by the network in an attempt to rebuild the London agency.
Lowe has suffered a steady decline in the past five years with the loss of major business from General Motors, Coca-Cola, Unilever and Braun. The losses peaked at the end of 2005 when Tesco moved its £45m account.
However, in January Lowe signed up its executive creative director, Ed Morris, in a "golden handcuffs" deal that sees him contracted to the business until 2009. Then, in April, the agency appointed Rebecca Morgan, BT's head of marketing communications, to the newly created post of chief strategy officer.
Gatfield said: "Amanda's poise, confidence and credibility struck a powerful chord with Tony and myself. The combination of Amanda, Ed and Rebecca, supported by Chris Hunton [the managing director] and Judi Mitchem [the chief marketing officer], is exceptional."
Walsh added: "The agency has a great team in place and my job is to be their leader and make them feel confident about themselves."
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