Miller, out of jail after 85 days, said: "I was a journalist doing my job, protecting my source until my source freed me to perform my civic duty to testify."
Escorted by her lawyers and New York Times publisher Arthur Sulzberger Jr, Miller met reporters for several minutes on Friday after spending more than four hours inside the courthouse, most of it behind closed doors with a grand jury.
Miller said she agreed to meet the grand jury after hearing "directly from my source" by telephone and in a letter that she should cooperate with the investigation by prosecutor Patrick Fitzgerald.
As part of the deal, Fitzgerald agreed in advance that he would limit Miller's testimony to her communications with her source "and that was very important to me", Miller added.
"I know what my conscience would allow and ... I stood fast to that," the reporter said.
Miller's testimony has been characterised by Fitzgerald as key to his investigation into the White House role in the disclosure of Plame's identity.
"I was a journalist doing my job, protecting my source until my source freed me to perform my civic duty to testify"
New York Times reporter
Although Miller declined to identify her source, The New York Times identified him as Lewis "Scooter" Libby, chief of staff to Vice-President Dick Cheney.
Until a few months ago, the White House had maintained for nearly two years that Libby and presidential aide Karl Rove were not involved in leaking the identity of Plame, whose husband had publicly suggested that the Bush administration twisted intelligence in the run up to the war in Iraq.
The timing of the criticism by former ambassador Joseph Wilson was devastating for the White House, which was already on the defensive because no weapons of mass destruction had been found in Iraq.
The president's claims of such weapons were the main justification for going to war.
Miller (C) testified on Friday
before a grand jury
Libby met Miller just two days after Wilson blasted the Bush administration in a Times op-ed piece.
Time magazine reporter Matthew Cooper testified recently that Rove and Libby had spoken to him about Wilson's wife that same week in July 2003 when Miller spoke to Libby.
In October 2003, with the criminal investigation gaining speed, White House spokesman Scott McClellan said of Rove and Libby: "Those individuals assured me they were not involved in this (leaking of Plame's identity)."
Miller, released from jail on Thursday night, had been in custody in Alexandria since 6 July.
A federal judge ordered her jailed for civil contempt of court when she refused to testify.
The disclosure of Plame's identity by syndicated columnist Robert Novak on 14 July 2003 triggered the criminal investigation that could still result in charges against government officials.