Ballmer made his decision public a month after his successor, Satya Nadella, announced unprecedented job cuts at the U.S. technology firm as it worked to adapt to a "mobile first, cloud first" world.
"Given my confidence and the multitude of new commitments I am taking on now, I think it would be impractical for me to continue to serve on the board, and it is best for me to move off," Ballmer said in a publicly posted letter to Nadella. "I bleed Microsoft—have for 34 years and I always will."
The company said it would slash 18,000 jobs from its global workforce over the next year, the majority from the Nokia handset unit acquired this year.
Nadella, who became CEO earlier this year, seeks to reinvigorate Microsoft, which had been the world's largest technology company but has lagged in recent years as Google and Apple (IW 500/4) (AAPL) have taken leadership of the tech sector.
Copyright Agence France-Presse 2014