Warner takes the role of global brand director, which has been vacant since Dee Dutta was promoted to vice-president of global marketing last year.
Warner, who will report to Dutta, had been responsible for Microsoft's digital marketing for its key consumer brands, including Windows and Office.
He was previously marketing director of MSN Carview and head of marketing for Emap Digital.
Warner will be responsible for developing the brand globally and growing Sony Ericsson's market share, which stands at 5%, according to Strategy Analytics. He will also liaise with the firm's main agencies, including Bartle Bogle Hegarty and Dare.
His appointment comes as Sony Ericsson gears up to boost promotional spend on its handset range in a bid to establish itself as the leader of the camera-phone market.