Here are seven major takeaways from his testimony.
1. Comey is pretty sure Trump inappropriately interfered in the investigation - but Trump didn't ask the FBI to drop it entirely
The way Comey understood his conversations with the president, Trump asked Comey for three things:
▪ His loyalty while appearing to threaten his job security
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▪ To "lift the cloud" of any perception the president was under investigation
▪ To drop the FBI's investigation into Trump's fired national security adviser Michael Flynn.
"The ask was to get it out that I, the president, am not personally under investigation," Comey said.
But, Comey testified, Trump did NOT ask him to drop the FBI's broader investigation into Russia meddling in the 2016 election and whether Trump's campaign helped.
Comey also declined to give a legal judgment on whether Trump obstructed justice or whether he colluded with Russia, saying that's up for the FBI and special counsel to investigate.
2. Comey thinks the president is a liar
The way Comey tells it, the first time he met Trump, Comey got the heebie-jeebies - for a whole bunch of small reasons but nothing in particular.
"I was honestly concerned he might lie about the nature of our meeting," Comey said, as to why he left Trump Tower, hopped in an FBI car, opened a laptop and started writing down every detail he could recall about his first meeting with the president. "It led me to believe that I gotta write it down, and I gotta write it down in a detailed way. I knew that there might come a day where I might need a record of what happened, not just to defend myself and FBI and the integrity of our situation, and the independence of our function."
Comey also said the president lied about why he fired him:
"The administration then chose to defame me - and, more importantly — the FBI by saying the organization was in disarray and that it was poorly led, that the workforce had lost confidence in its leader. Those were lies, plain and simple."
3. The way Trump handled Comey's firing is what prompted Comey to speak out
First, Comey found out he was fired by watching TV.
Second, Comey said he was confused about why he was fired. The president changed his narrative several times, ultimately settling on "that Russia thing." Then, Comey read in the press that the president told Russians Comey was a "nut job."
Finally, Trump tweeted this:
James Comey better hope that there are no "tapes" of our conversations before he starts leaking to the press!
Up until then, Comey said he and senior leaders in the FBI had decided to "keep . . . in a box" everything they had learned about the president's inappropriate questions about the investigation.
But after Trump's tweet, Comey said he couldn't stay silent.
"I woke up in the middle of the night Monday [thinking] that there might be corroboration for our conversation," Comey testified. "And my judgment was that I needed to get that out in the public square. So I asked a friend of mine to share the content of [my memos] with a reporter."
4. Democrats are pretty sure Comey's firing is the key to what the president did wrong
"I believe the timing of your firing stinks," said Sen. Ron Wyden, D-Ore.
Sen. Mark R. Warner, Va., the top Democrat on the Senate Intelligence Committee, called Comey's firing "ultimately shocking."
"As director of the FBI, Comey was ultimately responsible for conducting the investigation, which might explain why you're sitting down as a private citizen," Warner said.
Comey agreed that he thinks his firing was tied to the president's frustrations with how Comey was handling the Russia investigation.
"Something about the way I was conducting, it created pressure, and he wanted me to leave," Comey said.
5. Republicans aren't really trying to defend the president
As close as they got was one GOP senator trying to argue that: Okay, what Trump did was wrong, but is it really obstruction of justice?
"He said: 'I hope' [when he asked you to drop the Flynn investigation]," said Sen. James E. Risch, R-Idaho, a Trump ally. "You don't know of anyone that's ever been charged for hoping something?"
Comey said he didn't.
Sen. John Cornyn, R-Texas, made the point that the Trump's third ask - for Comey to "lift the cloud" by saying publicly the president was not under investigation - is a reasonable one.
Comey agreed but said that the president didn't seem to understand it could create a "boomerang effect" where if Trump ever was under investigation, the FBI would have to retract its public statement.
6. Republicans are critical of why Comey didn't speak up sooner
"The president never should have cleared the room," said Sen. Susan Collins, R-Maine, of a key Oval Office private meeting between Comey and Trump. "And he never should have asked you to let [the investigation into Flynn] go.
"But I remain puzzled by your response. Your response was: 'I agree that Michael Flynn was a good guy.' You could have said: 'Mr. President, this meeting is inappropriate, this response could compromise the investigation.'"
Comey testified that he was "stunned" the president was asking him to drop an investigation and, in retrospect, he probably should have been more firm with the president. But he just wanted to say something - anything - to end the "awkward" conversations.
And, Comey said, he doesn't regret keeping the president's conversations within a tight circle: "No action was the most important thing I could do to make sure there was no interference in the investigation."
7. No side comes off well in Comey's telling of events
To hear Comey tell it, when Republicans are in charge and the FBI was investigating Republicans, he was pressured by Republicans to shape his investigation.
And when Democrats were in charge and he was investigating Democrats, he was pressured by Democrats to shape his investigation. This is new - and significant. It suggests that no side was immune to meddling in the FBI's independent investigations.
Comey testified that when he was investigating Hillary Clinton's emails during the 2016 presidential campaign, Attorney General Loretta E. Lynch (a President Barack Obama appointee) "directed me not to call it an investigation but instead to call it a matter."
That, plus Lynch's private tarmac meeting with former president Bill Clinton ahead of the FBI's impending decision on whether Clinton may have criminally mishandled classified information, raised Comey's ethics radar and persuaded him to announce the FBI's findings ahead of schedule.
"That was one of the bricks in the load that led me to conclude: I have to step away from the department if we're to close this case credibly," Comey said.
Amber Phillips writes about politics for The Washington Post’s The Fix.