Walker comes from MindShare where she was global communications planning director on its Unilever account.
Walker has previously worked for Carat International as a global account director on Vodafone and for ZenithOptimedia in Australia as group account director on Nestle.
Graham Duff, president of Universal McCann EMEA, said: "Sarah was by far the strongest candidate, she has precisely the right balance of strategic insight, international experience and interpersonal skills."