WikiLeaks was founded in 2006. That year, Assange wrote two essays setting out the philosophy behind WikiLeaks: "To radically shift regime behavior we must think clearly and boldly for if we have learned anything, it is that regimes do not want to be changed. We must think beyond those who have gone before us and discover technological changes that embolden us with ways to act in which our forebears could not." In his blog he wrote, "the more secretive or unjust an organisation is, the more leaks induce fear and paranoia in its leadership and planning coterie. ... Since unjust systems, by their nature induce opponents, and in many places barely have the upper hand, mass leaking leaves them exquisitely vulnerable to those who seek to replace them with more open forms of governance."

Assange sits on Wikileaks's nine-member advisory board, and is a prominent media spokesman on its behalf. While newspapers have described him as a "director" or "founder" of Wikileaks, Assange has said, "I don't call myself a founder"; he does describe himself as the editor-in-chief of WikiLeaks, and has stated that he has the final decision in the process of vetting documents submitted to the site. Like all others working for the site, Assange is an unpaid volunteer Assange says that Wikileaks has released more classified documents than the rest of the world press combined: "That's not something I say as a way of saying how successful we are – rather, that shows you the parlous state of the rest of the media. How is it that a team of five people has managed to release to the public more suppressed information, at that level, than the rest of the world press combined? It's disgraceful." Assange advocates a "transparent" and "scientific" approach to journalism, saying that "you can't publish a paper on physics without the full experimental data and results; that should be the standard in journalism." In 2006, CounterPunch called him "Australia's most infamous former computer hacker." The Age has called him "one of the most intriguing people in the world" and "internet's freedom fighter." Assange has called himself "extremely cynical". The Personal Democracy Forum said that as a teenager he was "Australia's most famous ethical computer hacker." He has been described as being largely self-taught and widely read on science and mathematics, and as thriving on intellectual battle.

WikiLeaks has been involved in the publication of material documenting extrajudicial killings in Kenya, a report of toxic waste dumping on the African coast, Church of Scientology manuals, Guantánamo Bay procedures, the July 12, 2007 Baghdad airstrikes video, and material involving large banks such as Kaupthing and Julius Baer among other documents.