Bradley Manning, who leaked thousands of classified documents to WikiLeaks, has been convicted on 20 charges, including espionage.
However, the US soldier was cleared of the most serious charge of aiding the enemy and one which, if proved, could have had major ramifications for other whistleblowers.
The 25-year-old had admitted leaking the documents to the anti-secrecy organisation but insisted his actions had been intended to provoke a debate on US foreign policy.
He now faces up to 136 years in prison. Sentencing will begin on Wednesday.
The verdicts brought condemnation from human rights organisations, including Amnesty International and WikiLeaks.
WikiLeaks founder Julian Assange said the conviction set a "dangerous precedent".
Bradley Mannng could spend the rest of his life behind bars
The Australian, who has been inside the Ecuador Embassy for over a year to avoid extradition to Sweden, said he took no solace from Manning's acquittal on the most dangerous charge of aiding the enemy.
Mr Assange said the only victim in the case had been the US government's "wounded pride", adding that Manning's disclosures had helped spark the Arab Spring.
"This was never a fair trial," he told a small group of news agencies.
He criticised the treatment of the US soldier since his arrest in 2010, saying he had been stripped, kept isolated and in a cage to "break" him.
"WikiLeaks will not rest until he is free," he insisted.
The verdicts were read out by the judge overseeing Manning's military court martial, Colonel Denise Lind, who warned those in the packed courtroom - nearly all of them Manning supporters - that she would accept no disruptions.
Journalists watching the proceedings from a remote media room had no time to gauge Manning's reaction before the military cut off the live feed from the courtroom.
Press freedom campaigners had warned that if the soldier was convicted of aiding the enemy, the verdict would threaten to criminalise both journalists and their sources.
Amnesty International condemned the US government for refusing to investigate allegations highlighted through the leaks.
“The government’s priorities are upside down. The US government has refused to investigate credible allegations of torture and other crimes under international law despite overwhelming evidence," said Widney Brown, senior director of international law and policy at AI.
“The government’s pursuit of the ‘aiding the enemy’ charge was a serious overreach of the law, not least because there was no credible evidence of Manning’s intent to harm the USA by releasing classified information to WikiLeaks.
“It’s hard not to draw the conclusion that Manning's trial was about sending a message: the US government will come after you, no holds barred, if you're thinking of revealing evidence of its unlawful behaviour," she added.
Amongst the many items that Manning leaked to WikiLeaks was footage taken from an Apache helicopter of an attack in 2007 that killed 12 people in the Iraqi capital Baghdad, including a Reuters photographer.
There were also thousands of battlefield reports from Iraq and Afghanistan as well as secure communications between Washington state departments and embassies around the world
WikiLeaks said via its Twitter account that the verdict marked "dangerous national security extremism from the Obama administration."
Mr Assange said the aiding an enemy charge was "absurd", put forward as a "red herring" to detract from the other charges.
He described the soldier as the best journalistic source the world had ever seen, uncovering war crimes in Iraq which he maintained had led to the removal of US troops from that country.
His disclosures had led to a "backlash" against the US government and had been followed by the leaking of sensitive information by Edward Snowden about the US's surveillance.
Asked about the role of his whistleblowing website in the Manning case, Mr Assange said: "We are pleased that throughout this case no evidence was produced from WikiLeaks against Bradley Manning.
"The allegation against him is that he spoke to a US informer who turned him in. Our processes have been successful.
"It is of great concern to us to see any national security source victimised, but we have chosen not to enter into a debate over whether he is one of our sources."
Mr Assange said the only just outcome would have been acquittal on all the charges, saying conviction was a "clear abuse" of the First Amendment and Espionage Act in the US.
He described the actions of Manning as "unquestionably heroic".
The convictions bring to an end a three-year period following the private's arrest in May 2011. Since then, he has been living in conditions in solitary confinement that a UN investigator compared to torture. The reality now is that he is likely to spend the rest of his life behind bars.