Gilmartin will become the seventh active Microsoft board member. He is not replacing a retiring director. Instead, Microsoft is expanding the board, a representative said. Historically, Microsoft's board has varied in size between five and eight members.
"We are very pleased to have Ray joining our board," Microsoft Chairman Bill Gates said in a statement. "Ray takes a long-term perspective on business strategy and understands what is required for success in a very competitive marketplace. He is also known for his work to improve health care for people worldwide."
Gilmartin has run Merck since June 1994. Before that, he served as chairman and CEO of Becton Dickinson, which manufactures medical supplies. He also sits on the boards of several health care organizations.
Other Microsoft board members include Steve Ballmer, Microsoft CEO; Ann McLaughlin, chairman emeritus of The Aspen Institute; and W.G. Reed, retired chairman of Simpson Investment Co.