Dewar replaces Lee Blyth, who recently left the role to join mobile ad network InMobi as head of sales for the London office.
Dewar joined Microsoft in 2006 as sales manager and has held his most recent role of group account director for three years.
In his new role, Dewar will be responsible for all mobile ad sales across Microsoft Advertising properties including the MSN mobile browser, ads in app and Skype mobile.
He will report directly to Andy Hart, general manager of Micrsoft UK Advertising and Online.
His appointment comes after a turbulent year for the internet and technology giant, which underwent a global reorganisation of the business.
As part of the UK restructure, Andy Hart was appointed to head up the enlarged Advertising and Online division in July, while Ashley Highfield, former managing director of the now defunct consumer and online division departed shortly after to join Johnston Press.