The testimony appeared to bolster the case of Trump critics who believe that the President may have obstructed justice and abused his power in his dealings with Comey, who he later fired.
He wrote that Trump said: "'I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go."
"I replied only that 'he is a good guy.'" Comey wrote, describing a private meeting with Trump in the Oval Office on February 14, then added: "I did not say I would 'let this go.'"
The exchange took place after a meeting between Trump and senior intelligence and homeland security officials, after which the President asked to speak to the FBI Director alone.
Comey said in his testimony that he understood the President to be requesting that he drop the investigation into Flynn, who had resigned the day before. But he says he did not understand Trump to be referring to the wider Russia investigation.
"Regardless, it was very concerning, given the FBI's role as an independent investigative agency."
Trump critics contended that this encounter appears to be tantamount to an inappropriate pressure on the FBI by the President, an allegation that if proven could have dire consequences for Trump's presidency itself.
Comey's account of this encounter conflicts with Trump's own statements. At a press conference on May 18, the President was asked whether he had asked the FBI Director to pull the plug on the Flynn component of the Russia investigation.
"No, No, next question," Trump said.
"There is a criminal investigation going on of one of the President's top associations ... he gets fired, he is under under investigation and the President brings in the FBI Director and says 'please stop your investigation,'" said CNN's senior legal analyst Jeffrey Toobin.
"If that isn't obstruction of justice, I don't know what is," Toobin said.
But Trump's defenders were quick to seize on the document as well, arguing that it supported Trump's claims that the former FBI chief had told him three times
that he was not personally being investigated in the Russia probe.
Comey wrote in his testimony that he twice told Trump he was not being investigated once before the inauguration and once when he was President and also said that he had told congressional leaders that the FBI was not "personally investigating President Trump."
Trump's camp argued that fact vindicated the President and sought to use it to sweep away the Russia questions that have been clouding his White House ever since the inauguration.
"The President is pleased that Mr. Comey has finally publicly confirmed his private reports that the President was not under investigation in any Russian probe," said Marc Kasowitz, Trump's personal attorney.
"The President feels completely and totally vindicated. He is eager to continue to move forward with his agenda."
Trump's lawyer Michael Cohen told CNN, "Comey's statement released today needs to be carefully scrutinized as his testimony claims the president was concerned about the dossier."
The Republican Party also singled out the same helpful passages of Comey's testimony.
"President Trump was right," said Republican National Committee Chairwoman McDaniel.
"Director Comey's statement reconfirmed what the president has been saying all along -- he was never under investigation," McDaniel said in a statement that did not mention the other aspects of Comey's testimony.
The testimony was posted without notice on the website of the Senate Intelligence Committee, instantly electrifying Washington, which has been on edge for days ahead of Comey's planned testimony.
The dramatic intervention was classic Comey: the towering FBI chief, branded a "showboat" by Trump, has a reputation for theatrical public coups, and his move will only intensify the anticipation for his appearance on Thursday.
Comey described a March 30 phone call in which he said Trump stressed "the cloud" of the Russia investigations was "interfering with his ability to make deals for the country and said he hoped I could find a way to get out that he wasn't being investigated."
But Comey determined that to offer such an assurance would be unwise, not least because it would have to be corrected should the situation change.
The former FBI chief also wrote a revealing description of Trump's efforts to win his loyalty during their first dinner in January.
Trump, Comey wrote, told him, "I need loyalty, I expect loyalty" during their first dinner in January. Comey said in the statement, "I didn't move, speak or change my facial expression in any way during the awkward silence that followed." Comey replied, "you will always get honesty from me." He said the President responded, "that's what I want. Honest loyalty."
Comey said that before Trump brought up the issue of loyalty he had told the President he was not "reliable" in the ways politicians use the word but that he could count on him to tell the truth.
"I added that I was not on anybody's side politically and could not be counted on in the traditional political sense, a stance I said was in his best interest as the President," Comey wrote.
The former FBI director said that he had nine separate conversations with Trump, three of which were in person and six were on the telephone. By comparison, he said he spoke twice with President Barack Obama, and never on the telephone. He said that after meeting Trump he immediately begin to write notes about his conversations with Trump, a practice he didn't adopt before.
Comey offered intimate details of his encounter with Trump in an apparent attempt to create added authenticity to his account.
He said they dined alone on January 27 in the Green Room of the White House at a small oval table and were waited upon by two navy stewards.
He wrote that his instincts told him that the one-on-one setting and the tone of the conversation meant that Trump was seeking to get him to ask to remain in his job, in an attempt to "create some sort of patronage relationship."
"That concerned me greatly, given the FBI's traditionally independent status in the executive branch," Comey wrote. He added that he wrote a memo about the meeting and shared it with the senior leadership of the FBI.
Reaction to Comey's dramatic testimony was swift, reflecting the political tsunami that is raging over the Russia issue and that has often seemed about to swamp Trump's administration.
Republican Sen. John McCain told CNN's Manu Raju that he found the testimony "disturbing."
Rep Adam Schiff, the top Democrat on the House Intelligence Committee, said that Comey's testimony "confirms a host of troubling allegations concerning the President's conduct."
But Matt Schlapp, Chairman of the American Conservative Union played down the impact of Comey's testimony.
"This is one man's account of a conversation, it is a set of notes, it's exactly what we expect from James Comey. ... He is a grand-stander, he likes to be the honest man, he is going to play that role on Thursday," Schlapp said, reflecting the emerging GOP attack against the former FBI director as someone who cannot resist the spotlight.
Schlapp told CNN that only the judgment of the Special Counsel Robert Mueller will establish the facts of whether there was any collusion between the Trump campaign and Russia's effort to hack the election.
He also said it was "absolutely human nature to want to know you are a subject and Donald Trump was told from the very beginning of this that he was not."