At Westminster Magistrates’ Court on Thursday he was found guilty of failing to surrender to the court.
He now faces US federal conspiracy charges related to one of the largest ever leaks of government secrets.
The UK will decide whether to extradite Assange, in response to allegations by the Department for Justice that he conspired with former US intelligence analyst Chelsea Manning to download classified databases.
He faces up to five years in US prison if convicted on the charges of conspiracy to commit computer intrusion.
Assange’s lawyer Jennifer Robinson said they would be fighting the extradition request. She said it set a “dangerous precedent” where any journalist could face US charges for “publishing truthful information about the United States”.
She said she had visited Assange in the police cells where he thanked supporters and said: “I told you so.”
Assange had predicted that he would face extradition to the US if he left the embassy.
After his arrest, the 47-year-old Australian national was initially taken to a central London police station before appearing in court.
Dressed in a black suit and black polo shirt, he waved to the public gallery and gave a thumbs up. He pleaded not guilty to the 2012 charge of failing to surrender to the court.
Finding him guilty of that charge, District Judge Michael Snow said Assange’s behaviour was “the behaviour of a narcissist who cannot get beyond his own selfish interest”.
He sent him to Southwark Crown Court for sentencing, where he faces up to 12 months in prison.
The court also heard that during his arrest at the embassy he had to be restrained and shouted: “This is unlawful, I am not leaving.”
Assange set up Wikileaks in 2006 with the aim of obtaining and publishing confidential documents and images.
The organisation hit the headlines four years later when it released footage of US soldiers killing civilians from a helicopter in Iraq.
Former US intelligence analyst Chelsea Manning was arrested in 2010 for disclosing more than 700,000 confidential documents, videos and diplomatic cables to the anti-secrecy website.
She said she only did so to spark debates about foreign policy, but US officials said the leak put lives at risk.
She was found guilty by a court martial in 2013 of charges including espionage. However, her jail sentence was later commuted.
Manning was recently jailed for refusing to testify before an investigation into Wikileaks’ role in revealing the secret files.
The indictment against Assange, issued last year in the state of Virginia, alleges that he conspired in 2010 with Manning to access classified information on Department of Defense computers. He faces up to five years in jail.
Manning downloaded four databases from US departments and agencies between January and May 2010, the indictment says. This information, much of which was classified, was provided to Wikileaks.
The US Justice Department described it as “one of the largest compromises of classified information in the history of the United States”.
Cracking a password stored on the computers, the indictment alleges, would have allowed Manning to log on to them in such a way as to make it harder for investigators to determine the source of the disclosures. It is unclear whether the password was actually broken.
Correspondents say the narrowness of the charge seems intended to avoid falling foul of the US Constitution’s First Amendment guarantee of freedom of the press.
The Wikileaks co-founder had been in the Ecuadorian embassy in London since 2012, after seeking asylum there to avoid extradition to Sweden on a rape allegation.
The investigation into the alleged rape, which he denied, was later dropped because he had evaded the arrest warrant. The Swedish Prosecution Authority has said it is now considering whether to resume the inquiry before the statute of limitations runs out in August 2020.
Scotland Yard said it was invited into the embassy on Thursday by the ambassador, following the Ecuadorian government’s withdrawal of asylum.
Ecuadorian president Lenin Moreno said the country had “reached its limit on the behaviour of Mr Assange”.
Mr Moreno said: “The most recent incident occurred in January 2019, when Wikileaks leaked Vatican documents.
“This and other publications have confirmed the world’s suspicion that Mr Assange is still linked to WikiLeaks and therefore involved in interfering in internal affairs of other states.”
His accusations against Assange also included blocking security cameras at the embassy, accessing security files and confronting guards.
Mr Moreno said the British government had confirmed in writing that Assange “would not be extradited to a country where he could face torture or the death penalty”.
The arrest comes a day after Wikileaks said it had uncovered an extensive spying operation against its co-founder at the Ecuadorian embassy.
There has been a long-running dispute between the Ecuadorian authorities and Assange about what he was and was not allowed to do in the embassy.
BBC diplomatic correspondent James Landale said that over the years they had removed his access to the internet and accused him of engaging in political activities – which is not allowed when claiming asylum.
He said: “Precisely what has happened in the embassy is not clear – there has been claim and counter claim.”
Prime Minister Theresa May told the House of Commons: “This goes to show that in the UK, no one is above the law.”
Foreign Secretary Jeremy Hunt said the arrest was the result of “years of careful diplomacy” and that it was “not acceptable” for someone to “escape facing justice”.
But Labour leader Jeremy Corbyn said that Assange had revealed “evidence of atrocities in Iraq and Afghanistan” and his extradition “should be opposed by the British government”.
Press freedom organisation Reporters Without Borders said that the UK should resist extradition, because it would “set a dangerous precedent for journalists, whistleblowers, and other journalistic sources that the US may wish to pursue in the future”.
Australia’s Foreign Minister Marise Payne said he would continue to receive “the usual consular support” and that consular officers will try to visit him.
And actress Pamela Anderson, who has visited the embassy to support Assange, said the arrest was a “vile injustice”.