Preston, who is currently director of marketing, portfolio and innovation for Molson Coors UK, will assume the duties while the company continues its hunt for a full-time marketing director.
He has been with the company in various marketing and sales roles for 18 years.
In his new role, Preston will be responsible for a £70m group marketing budget, which includes the live review of the group's £15m media duties handled by Aegis Media's Vizeum.
Other agencies battling for the business are WPP's MediaCom and Mindshare, and Publicis Groupe's ZenithOptimedia.
Preston will now report directly to chief executive Mark Hunter, who is also a previous incumbent of the marketing director position.
Outgoing marketing director Davies leaves Molson Coors on a high, with flagship brand Carling's new ad campaign, "You know who your mates are", winning gold and silver British Television Advertising awards.
Davies is leaving to set up his own creative services agency called MMIXX.
Molson Coors has bucked the market trend in the past two years and increased its marketing investment through the recession. It also recently acquired Cobra and Singha.
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