The material she exposed included field reports from Iraq and Afghanistan, cables between the State Department and US embassies and assessments of detainees held at Guantanamo Bay, Cuba.
During Manning's 2011 military trial, prosecutors revealed chat logs showing the army private chatted with Assange about cracking a password to a computer anonymously. But during the trial Manning testified she acted alone and approached other news organisations before going to WikiLeaks. The anti-secrecy website published the material.
In a statement last week, Manning said she stood by that testimony and sees no reason to repeat it.
Outside the courthouse today, Manning did not mention Assange or WikiLeaks, saying she opposed grand juries as a principle.
"Grand juries are terrible tools. The idea that there is an independent grand jury is long gone; it's run by a prosecutor."
Witnesses do not have their own lawyer in the secret proceedings, she noted: "There is no adversarial process . . . I am generally opposed to the existence of a grand jury."
"There was an awful lot of government attorneys in there," she told a crowd of reporters and activists. But, she said, "we didn't learn anything" about why the government had subpoenaed her now, years after her conviction. "I only can speculate," she said.
She was represented in court by Moira Meltzer-Cohen, Sandra Freeman and Chris Leibig.
Prosecutors inadvertently exposed that Assange has been charged under seal late last year, but the nature of the charges against him remain unknown. US officials speaking on the condition of anonymity because of grand jury secrecy say the case is based on his pre-2016 conduct, not the election hacks that drew the attention of Special Counsel Robert Mueller.
Assange has for the past seven years been living in asylum at Ecuador's London Embassy, after a warrant was issued for his arrest in a Swedish sexual assault investigation. That case has been dropped, but Assange is still wanted by British authorities for skipping bail and has resisted leaving the embassy for fear of extradition to the United States.
Since leaving prison Manning, who was formerly known as Bradley Manning, has become an activist for transgender rights and launched an unsuccessful primary campaign against Senator Benjamin Cardin, D.
A group of supporters cheered as Manning left the courthouse, shouting "We love you Chelsea!" and waving signs saying, "Solidarity with Chelsea" and "Defend Grand Jury Resistance."
