District judge Vanessa Baraitser highlighted the intense restrictions and isolated conditions he would be likely to face in the US, saying they mean extradition would be “oppressive”.
But she said Assange would be kept in custody, ahead of an appeal expected from the US.
Edward Fitzgerald QC, representing Assange, said that in light of the judgment, “there are the strongest grounds for granting bail”, and a full application will be made on Wednesday.
Supporters gathered outside London’s Old Bailey ahead of Monday’s hearing, as journalists and cameras from media outlets around the world lined the street.
Assange appeared in the dock wearing a navy blue suit and dark green face mask, speaking only to give his name and date of birth during the hearing.
He wiped his brow after the decision was announced while his fiancée, Stella Moris, with whom he has two young sons, broke down in tears.
She was embraced by the editor-in-chief of WikiLeaks, Kristinn Hrafnsson, who sat next to her in court as the judgment was delivered.
The prosecution have given notice that the decision will be appealed and have 14 days to lodge their grounds.
The case follows WikiLeaks’ publication of hundreds of thousands of leaked documents in 2010 and 2011 relating to the Afghanistan and Iraq wars, as well as diplomatic cables.
The judge said Assange’s extradition would be “oppressive” because of his mental health and the conditions he would face under “special administrative measures” in US detention, and at the ADX Florence prison.
She told the court that he would be in “conditions of near total isolation” and without the factors needed to moderate his suicide risk, meaning that the extradition can be barred under section 91 of the Extradition Action 2003.
The law states that when “the physical or mental condition of the person is such that it would be unjust or oppressive to extradite him, the judge must order the person’s discharge”.
The court heard that Assange has been held at HMP Belmarsh in London since April 2019, and has been under a care plan for prisoners at risk of suicide or self-harm for the duration of his imprisonment.
Medical notes record numerous occasions of him telling a prison psychologist and other medical staff that he has suicidal or self-harming thoughts, felt despairing or hopeless, and had plans to end his life, the judge said.
Assange has contacted the Samaritans phone service on several occasions and a razor blade was found in his cell in 2019.
Judge Baraitser said she had accepted experts’ findings that Assange suffers from a recurrent depressive disorder, which is sometimes accompanied by psychotic features.
She said she also accepted the opinion that Assange suffers from autism spectrum disorder, “albeit high-functioning”, and Asperger’s syndrome.
The judge found that although Assange had demonstrated a lighter spirit at times, the “overall impression is of a depressed and despairing man who is genuinely fearful about his future”.
She opened her judgment by dismissing numerous arguments made by his defence team during the long-running proceedings.
She said that Assange’s extradition would not amount to a violation of freedom of speech, and that his role in disclosures made by Chelsea Manning “went beyond the mere encouragement of a journalist”, detailing technical assistance.
The judge said Assange had been attempting to recruit hackers “for some time” and that his actions “took him outside any role of investigative journalism”, adding: “He was acting to further the overall objective of WikiLeaks, to obtain information through hacking.”
The court heard that more than 100 people had been placed at risk by disclosures that named informants, and that some had to flee their home countries with their families for safety.
The judge said Mr Assange did not have “the right to sacrifice the safety of these individuals, knowing nothing of the circumstances or the dangers they face, in the name of free speech”.
She concluded that his actions would have been offences in England and Wales, including under the Official Secrets Act 1989.
Judge Baraitser found that the US prosecution was brought in “good faith”, and that there was insufficient evidence that prosecutors were pressurised by the Trump administration.
She said there was “healthy internal debate” in the US and highlighted Donald Trump’s previous praise of WikiLeaks, adding: “There is little or no evidence to support hostility by President Trump towards Mr Assange and WikiLeaks.”
The judge also concluded that the passage of time since the alleged offences did not make the case unjust, because the material in the case had posed “difficult problems for investigators”, and Mr Assange’s defence had not been hampered.
She called defence arguments that a jury pool in the US would be comprised almost entirely of government employees or contractors “untenable”.
Prosecutors say Assange helped US defence analyst Manning breach the Espionage Act in unlawfully obtaining material, was complicit in hacking by others, and published classified information that put the lives of US informants in danger.
Assange denies plotting with Manning to crack an encrypted password on US Department of Defence computers and says there is no evidence that anyone’s safety was put at risk.
Assange’s lawyers had said he faced up to 175 years in jail if convicted, although the US government said the sentence was more likely to be between four and six years.