him over 100 years, according to legal experts and WikiLeaks.
Sentencing arguments begin on Wednesday.
Manning, 25, was charged with 22 counts in connection with the leaking of classified US military and state department documents to the WikiLeaks website.
The verdict could impact leaks and whistleblowers generally, specially Edward Snowden, whose leaks, the government has claimed, made al Qaeda change its tactics.
The New York Times said the verdict "could have significant long-term ramifications for investigative journalism in the Internet era".
READ: WikiLeaks on Manning verdict: 'Extremism'
WikiLeaks agreed, saying in a tweet the judgment set "a very serious new precedent for supplying information (to) the press".
Supporters of US Army Pfc. Bradley E Manning hold signs to show support during a demonstration outside the main gate of Ft. Meade in Maryland. (AFP Photo)
Over 3,500 state department cables were about India originating either from the three US missions in New Delhi, Kolkata and Chennai or the State Department here in DC.
The leaked cables -- containing observations by US diplomats -- were embarrassing to all but little or no damage to relations between the two countries.
The most serious charge against Manning was "aiding the enemy," punishable with a life sentence. The prosecution had not pressed capital punishment, which was an option.
He pleaded guilty to lesser charges in March.
Manning was arrested on arrest in May 2010, for leaking over 700,000 documents he had accessed while serving as an intelligence officer with the US army in Iraq.
During closing arguments Manning's lawyer David Coombs had called him a well-intentioned man who tried to "ply his knowledge to hopefully save lives".
The chief prosecutor Major Ashden Fein told the judge, "He was a traitor, a traitor who understood the value of compromised information in the hands of the enemy and took deliberate steps to ensure that they, along with the world, received it."
A judge -- Army judge Col Denise Lind -- decided the case, not a jury, at Manning's request.
WikiLeaks began publishing information leaked by Manning in 2010. The first revelation about a video showing the US crew of a helicopter laughing while they gunned down a dozen people in Baghdad in July 2007.
Then came the Afghan war papers showing US concerns over Pakistani help for the Taliban, particularly the role of the Inter-Services Intelligence (ISI).
Private First Class Bradley Manning, 25, is escorted out of court, after Judge Colonel Denise Lind announced she would read her verdict the next day, at Fort Meade, Maryland. (Reuters Photo)
WikiLeaks then released in November 2010 a tranche of more than 250,000 US state department cables, dating from December 28, 1966 to February 28, 2010.
That set off a storm in capitals all over the world.
The United States publicly refused to either explain or defend the offensive observations or remarks in the cables, but despatched officials to put out the fire.
But it was not entirely successful in containing the fallout.
Some people argue that US cables about rampant corruption in the Tunisian government may have triggered the unrest that spread elsewhere becoming what is now called Arab Spring.