Jurors found Jeffrey Sterling, 47, guilty on all nine counts he faced in federal court in Alexandria, Virginia.
The conviction marked a victory of sorts for President Barack Obama's administration in its crackdown on whistleblowers. It has used the nearly century-old Espionage Act to prosecute government officials suspected of leaking classified data.
"This is a just and appropriate outcome," Attorney General Eric Holder said in a statement.
"The defendant's ... disclosures placed lives at risk. And they constituted an egregious breach of the public trust by someone who had sworn to uphold it."
The case has dragged in court for years as prosecutors pressed Times journalist James Risen to reveal his sources. Risen was subpoenaed in 2008 and again in 2011 ordering him to testify at Sterling's trial.
US Attorney Dana Boente noted that classified information is "critical to our national defense, and releasing it was illegal and went against Mr Sterling's professional commitments to the CIA."
His "vindictive and careless choices ultimately led us here today and to this unanimous verdict," Boente said.
Sterling was not allowed to share secret information with unauthorized people, including the media.
Sterling's lawyer, Edward MacMahon Jr, told The New York Times he would try to get the verdict dismissed and if that failed, he would file an appeal.
"We continue to believe in Jeffrey's innocence, and we're going to continue to fight for him up to the highest levels."
From November 1998 through May 2000, Sterling was assigned to a classified clandestine operational program designed to undermine the Iranian nuclear weapons program, officials said.
He was reassigned in May 2000 and later that year he began civil actions against the CIA.
"In retaliation for the CIA's refusal to settle those actions, (Sterling) disclosed information concerning the classified operational program," prosecutors said.
Prosecutors ultimately dropped their attempts to call the Pulitzer Prize-winning journalist Risen to testify after it became clear he would not reveal his sources, even if jailed, for his account of a bungled CIA operation in Iran that appeared in his 2006 book, "State of War."
US District Judge Leonie Brinkema ordered Sterling released pending his sentencing hearing on April 24.
The case has sparked outcry among media watchdogs, with more than 100,000 supporters signing an online petition delivered to the US Justice Department calling for an end to the prosecution.