Andreessen was born in Cedar Falls, Iowa and raised in New Lisbon, Wisconsin. He is the son of Patricia and Lowell Andreessen, who worked for a seed company. In December 1993, he received his bachelor's degree in computer science from the University of Illinois at Urbana–Champaign. As an undergraduate, he interned twice at IBM in Austin, Texas. He worked in the AIX graphics software development group which was responsible for the MIT X-windows implementation and ports of the 3D language API's: SGI's Graphics Language (GL) and PHIGS. He also worked at the National Center for Supercomputing Applications (NCSA) at the University of Illinois, where he became familiar with Tim Berners-Lee's open standards for the World Wide Web. Andreessen and full-time salaried co-worker Eric Bina worked on creating a user-friendly browser with integrated graphics that would work on a wide range of computers. The resulting code was the Mosaic Web browser.
"In the Web's first generation, Tim Berners-Lee launched the Uniform Resource Locator (URL), Hypertext Transfer Protocol (HTTP), and HTML standards with prototype Unix-based servers and browsers. A few people noticed that the Web might be better than Gopher.
In the second generation, Marc Andreessen and Eric Bina developed NCSA Mosaic at the University of Illinois. Several million then suddenly noticed that the Web might be better than Gopher.
In the third generation, Andreessen and Bina left NCSA to found Netscape..."
— Bob Metcalfe, InfoWorld, August 21, 1995, Vol. 17, Issue 34.
After his graduation from UIUC in 1993, Andreessen moved to California to work at Enterprise Integration Technologies. Andreessen then met with Jim Clark, the founder of Silicon Graphics, who had recently exited the firm. Clark believed the Mosaic browser had great commercial possibilities and suggested starting an Internet software company. Soon, Mosaic Communications Corporation was in business in Mountain View, California, with Andreessen as co-founder and vice president of technology. The University of Illinois was unhappy with the company's use of the Mosaic name, so Mosaic Communications changed its name to Netscape Communications, and its flagship Web browser was the Netscape Navigator.
In the year between the formation of the company and its IPO, Andreessen engaged in extensive public outreach on behalf of his vision of the Web browser's potential, something he had in fact done continuously since making the decision to distribute Mosaic for free via the Internet.
One of these events, hosted by Internet commercialization pioneer Ken McCarthy, was captured on video and provides a unique look at the state of the Web between the time Andreessen and his colleagues launched Mosaic and the time when Web browsers and servers became mainstream commercial products. At the time of the recording, Andreessen was 23 years old.
Netscape's IPO in 1995 propelled Andreessen into the imagination of the public. Featured on the cover of Time and other publications, Andreessen became the poster-boy wunderkind of the Internet bubble generation: young, twenty-something, high-tech, ambitious, and worth millions (or billions) of dollars practically overnight.
Netscape's success attracted the attention of Microsoft, which recognized the Web's potential and wanted to put itself at the forefront of the rising Internet revolution. Microsoft licensed the Mosaic source code from Spyglass, Inc., an offshoot of the University of Illinois, and turned it into Internet Explorer. The resulting battle between the two companies became known as the Browser wars. In 1997, Andreessen was featured on the cover of Interactive Week Magazine.
Netscape was acquired in 1999 for $4.2 billion by AOL, which made Andreessen its Chief Technology Officer. The same year, he was named to the MIT Technology Review TR100 as one of the top 100 innovators in the world under the age of 35.
Andreessen left AOL to form Loudcloud with Ben Horowitz, Tim Howes, and In Sik Rhee. Loudcloud was a services-based Web hosting company that made an IPO in 2001. Loudcloud sold its hosting business to EDS and changed its name to Opsware in 2003, with Andreessen serving as chairman. Opsware was purchased by Hewlett-Packard in September 2007 for about $1.6 billion.
Between 2005 and 2009, Andreessen and longtime business partner Ben Horowitz separately invested a total of $4 million in 45 start-ups that included Twitter and Qik. The two became well known as super angel investors. On July 6, 2009, Andreessen and Horowitz announced their Silicon Valley venture-capital firm Andreessen Horowitz. The firm invests in entrepreneurs, products, and companies in the information technology industry.
Begun with an initial capitalization of $300 million, within three years the firm grew to $2.7 billion under management across three funds. Andreessen Horowitz’s portfolio holdings include Facebook, Foursquare, GitHub, Pinterest, Twitter, and Honor, Inc..
On September 1, 2009, an investor group that included Andreessen Horowitz acquired a majority stake in Skype for $2.75 billion, which was considered risky. The deal paid off in May 2011 when Microsoft bought Skype for $8.5 billion. Additionally, Andreessen and Horowitz made personal investments in headset maker Jawbone in 2006. The firm announced a $49 million investment in Jawbone in March 2011. In February 2011, Andreessen Horowitz’s $80 million investment in Twitter made it the first venture firm to hold stock in all four of the highest-valued, privately held social media companies (at that time): Facebook, Groupon, Twitter, and Zynga.
Andreessen's experiences as an entrepreneur, investor, and board member at several large technology firms have positioned him to “draw insightful conclusions” about technology trends and help Andreessen Horowitz “stand out from the crowd” (The Economist). Andreessen often advises the leaders of companies in which Andreessen Horowitz invests, including Mark Zuckerberg of Facebook and Mark Pincus of Zynga.
In 2010, notable Silicon Valley attorney Ted Wang created the first free standardized seed round financing documents, the Series Seed Documents.
An op-ed piece Andreessen authored for The Wall Street Journal in August 2011, “Why Software Is Eating the World”, received over 445 comments and prompted articles on the topic in The Economist and other media outlets. In the Journal, Andreessen wrote about the growing importance of software to businesses of all types:
More and more major businesses and industries are being run on software and delivered as online services—from movies to agriculture to national defense. Many of the winners are Silicon Valley-style entrepreneurial technology companies that are invading and overturning established industry structures. Over the next 10 years, I expect many more industries to be disrupted by software, with new world-beating Silicon Valley companies doing the disruption in more cases than not.
Andreessen and Horowitz were ranked no. six on Vanity Fair’s 2011 New Establishment List, no. 1 on CNET’s 2011 most influential investors list and no. 2 and 21, respectively, on the 2012 Forbes Midas List of Tech’s Top Investors.
In April 2012, Andreessen and Andreessen Horowitz General Partners Ben Horowitz, Peter Levine, Jeff Jordan, John O’Farrell, and Scott Weiss pledged to donate half of their lifetime incomes from venture capital to charitable organizations.
In 2012, Andreessen was named in the Time 100, an annual list of the 100 most influential people in the world assembled by Time.
In 2013, Andreessen was one of five Internet and Web pioneers awarded the inaugural Queen Elizabeth Prize for Engineering.
Andreessen cofounded and was chairman of Ning, the third company he established after Netscape and Loudcloud. In September 2011, it was announced that Ning had been sold to Mode Media for a reported price of $150 million. Andreessen joined Glam Media’s board of directors following the sale.
Andreessen is an investor in social news website Digg and several other technology startups such as Plazes, Netvibes, CastTV, Twitter, and the RockMelt browser launched in 2010. He is a personal investor in such diverse companies as LinkedIn and boutique bank Raine.
Andreessen serves on the board of Facebook, eBay, Hewlett Packard Enterprise, Kno, Stanford Hospital, Bump Technologies, Anki, Oculus VR, Dialpad, and TinyCo. He serves as advisor to Asana and director of CollabNet.
Andreessen is a proponent of Bitcoin and cryptocurrency and has described the technology as "innovative and radical". His venture capital firm Andreessen Horowitz has invested more than $227 million in bitcoin-related startups as of July 19, 2015.
Andreessen married Laura Arrillaga in 2006. She is the founder of the Silicon Valley Social Venture Fund and daughter of Silicon Valley real estate billionaire John Arrillaga.
In 2012, Andreessen expressed some support for Republican candidate Mitt Romney. During the 2016 primary season, he endorsed Republican candidate Carly Fiorina, but after Fiorina dropped out of the race, Andreessen switched his endorsement to the Democratic nominee Hillary Clinton, citing the Republican nominee Donald Trump's anti-immigration stance.
In February 2016, Marc Andreessen posted a tweet in response to India's decision to apply net neutrality to Facebook's proposed project Free Basics. The tweet suggested that anti-colonialism had been catastrophic for the Indian people. Andreessen later deleted the tweet following criticism from Indians and non-Indians alike (including Facebook founder Mark Zuckerberg). Facebook spent millions advertising Free Basics to the Indian public. The project failed due to violations, setting preferential tariffs in accessing content and setting up a "walled garden" on the internet.