But legal experts tell ABC News that it doesn't appear that Comey did anything illegal in sharing the information about his meetings with Trump.
What Comey said he did
During this afternoon's testimony, Comey said he shared the contents of a memo that he had previously written about a meeting he had had with Trump with a friend.
That decision, Comey said, came three days after Trump posted a tweet that read "James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!"
Comey said the tweet prompted his decision to tell his friend -- who was later confirmed by ABC News to be Columbia Law School professor Dan Richman -- about the memo in question.
"I woke up in the middle of the night on Monday night, because it didn't dawn on me originally, there might be corroboration, a tape," Comey said.
"My judgment was I needed to get that out into the public square. So I asked a friend of mine to share the content of the memo with a reporter. I didn't do it myself for a variety of reasons, but I asked him to because I thought that might prompt the appointment of a special counsel," Comey said.
How Trump's lawyer reacted
The president's personal lawyer, Marc Kasowitz, issued a statement after Comey's public testimony, reiterating facts that he felt bolstered Trump's claims and going after Comey for the revelation about how he shared his memos.
"Today, Mr. Comey admitted that he leaked to friends his purported memos of these privileged conversations, one of which he testified was classified," Kasowitz said this afternoon.
"We will leave it to the appropriate authorities to determine whether this leaks should be investigated along with all those others being investigated," he said.
Kasowitz made inaccurate claims about the timing of when Comey shared the memo and when the content of the memo was published in The New York Times. In Kasowitz's incorrect version, he said the information was published before Comey shared the memo, and therefore it appeared to be "entirely retaliatory."
In reality, Comey shared the memo with his friend days after Trump tweeted suggesting there may be tapes of their conversations, and the day after he shared the information, it was referenced by The New York Times.
Breaking down privileged information
Not everything the president says in private is automatically “privileged” and therefore unable to be shared freely, unlike attorney-client privilege or doctor-patient privilege. Executive privilege is not absolute.
As seen in the Supreme Court case against then-President Richard Nixon, courts weigh the interests for secrecy against the public interest when it comes to disclosure. The case, United States v. Nixon, took place during Nixon's impeachment proceedings and the court voted unanimously to order him to release tape recordings of his conversations.
Also, the president would have to claim executive privilege before his conversations with Comey and not now, after the fact, said Mark Rozell, author of "Executive Privilege: Presidential Power, Secrecy and Accountability." “It was incumbent upon Trump to make to clear to Comey that these conversations are privileged and off the record, perhaps to say ‘OK, let’s speak candidly here,’” said Rozell.
ABC News chief legal analyst Dan Abrams added that "the problem is [Trump] waived confidentiality on it by discussing these matters publicly previously."
"[Also], you can't use executive privilege to cover up alleged misconduct, and that's effectively what Comey is saying that he is doing here. So I think the notion that this is somehow privileged communications is simply not going to hold up," Abrams said.
Claims about classification
During his testimony, Comey specified that he purposefully wrote his memo to be unclassified following the Feb. 14 meeting with Trump -- wherein Trump allegedly dismissed everyone else to talk about national security adviser Michael Flynn with Comey alone.
"I remember thinking this is a very disturbing development, really important to our work. I need to document it and preserve it in a way that this committee gets this," Comey said, referencing the private conversation.
"Sometimes when things are classified, it tangles them up. It's hard to share it with an investigative team. You have to be careful how you handle it for good reason. My thinking was if I write it in such a way that I don't include anything that would trigger a classification. That will make it easier for us to discuss within the FBI and the government and to hold onto it in a way that makes it accessible to us."
Peter Schuck from Yale Law School told ABC News that Comey would have committed a crime in leaking the information to his friend who then shared it with a reporter "only if he was leaking classified information."
Anne Milgram, a former New Jersey attorney general and current law professor at New York University Law School, said that she thinks “the short answer is no” when it comes to whether Comey’s leak was illegal.
“Nothing Comey has said indicates that a crime was committed by his disclosure of the memos he wrote about his conversations with the president,” Milgram told ABC News.
Milgram also noted that Comey said the information shared was not classified, and “there is also no evidence that the memos were part of a grand jury investigation, which could trigger other prohibitions against disclosure.”
“According to Comey’s statements today, these were personal memos that he wrote to capture conversations, and his reaction to those conversations, around the time that they occurred,” Milgram added.
ABC News' Lauren Pearle contributed to this report.