Brown, who joined the company in 1989 as treasurer, will retire at the end of the calendar year. Maffei has served as vice president for corporate development since 1996 and as treasurer since 1994.
"Mike helped lead the company through important financial achievements: the development of global financial systems and our growth to $10 billion of revenue," said Bill Gates, chairman and chief executive officer, in a statement.
Prior to joining Microsoft, Brown was a partner at the public accounting firm of Deloitte & Touche, where he worked with Microsoft since 1979.
Tom Pilla, a company spokesman, would not comment on what Microsoft projects Brown would continue working on during his remaining months. Pilla did note that Brown would work with Maffei through the transition and devote time to the Nasdaq stock market, where he currently serves as chairman of the board.
Brown is known for his conservative accounting practices, and Pilla said the same style will continue under Maffei.
Analysts add that both individuals are very respected and that they are confident in Maffei's abilities, as he worked under Brown since 1994.
As CFO, Maffei will be responsible for Microsoft's worldwide finance organization, including corporate development, analyses, planning, accounting, reporting, treasury, taxes, investor relations, and corporate audits.
Since 1996, Maffei has been responsible for Microsoft's corporate development, planning, accounting, reporting, and treasury departments.