Andreessen, who is also a former chief technical officer at , joins Mark Zuckerberg, Facebook's founder, and venture capitalists Peter Thiel and Jim Breyer, on the social networking site's board.
Andreesen was one of those behind the development of Mosaic, which was the first major web browser that for a while rivalled Internet Explorer, and most recently co-founded Ning, a website that allows users to create their own social network with any design and in any language.
Andreessen said: " is one of the most innovative companies on the web and it is an honour to join the board. I am looking forward to helping the team as Facebook continues to grow."
The Silicon Valley veteran will remain with full time and retain his chairman title.
Zuckerberg said: "Marc is an industry leader, and we're fortunate to have him join our board.
"He has experience that is relevant to Facebook in so many ways: scaling companies that are experiencing extraordinary growth, creating successful technology platforms, and building strong engineering organizations.
"I know Marc will be a great mentor to me and our leadership team."
Andreessen is also an investor in social news website Digg and several social media start-ups including Netvibes and Twitter.