Ashmore, who joins from her role as global chief marketing capability officer at Mars, will take responsibility for Yell's global marketing strategy in the UK, US, Spain and Latin American markets.
She will report directly to group chief executive Mike Pocock. Prior to joining Mars, Ashmore had also held marketing roles at Procter & Gamble and Shell.
It is the latest in a series of senior marketing changes at Yell. In July, the company hired Bob Gregerson to the role of chief consumer officer and Chris Landry as group head of brand and design, tasked with enhancing its digital offering.
Yell has also hired brand consultancy Landor to carry out an evaluation of its brand portfolio, as it looks to transform itself into a primarily digital business.
Of particular focus is Yell's plan to become an "eMarketplace" online portal for SMEs to "connect and transact". The agency will be tasked with ensuring a "compelling, relevant and consistently implemented" brand identity in all global markets.
Pocock said: "As we continue to transform Yell's business to focus on delivery of digital marketing services to our 1.3 million SME customers, [Ashmore] will apply her considerable vision and insight to lead the definition, delivery and positioning of Yell's brand portfolio."