He was infamously sacked by Trump and in his first major TV interview since then, which aired on ABC News on Sunday, he claimed the 72-year-old is "morally unfit to be president".
That's the day Comey's book, "A Higher Loyalty", officially comes out.
ABC's George Stephanopoulos asked the former Federal Bureau of Investigation director whether Trump was "obstructing justice" when he asked Comey to let former National Security Adviser Mike Flynn off the hook.
Asked whether Trump was unfit to be president and should be impeached, Comey said people need to stand up and go to the voting booth and vote their values.
Comey answered "possibly" when asked whether the president was attempting to obstruct justice when he asked Comey to end an FBI investigation into former White House national security adviser Michael Flynn.
The book's imminent release - and the slated ABC News interview - prompted Trump to hurl a new set of insults at Comey earlier on Sunday, challenging accusations made in the book, and insisting that he never pressed Comey to be loyal to him. "He strikes me as a person of above average intelligence who's tracking conversations and knows what's going on", Comey said.
The FBI director, who until his firing last May led an investigation into possible ties between Russian Federation and the Trump campaign, acknowledged that it was "stunning" to think Russian Federation could have damaging information on an American president.
Trump had seized on an excerpt of the ABC interview shown Saturday in which Comey said his belief that Clinton would beat Trump in the 2016 presidential election was a factor in his decision to disclose the investigation into her emails. 'I think he's morally unfit to be president, ' Comey told Stephanopoulos.
In a separate interview with USA Today, Comey - in a reference to salacious allegations contained in an unverified dossier created by former British intelligence agent Christopher Steele - said it is hard "to explain some things without at least leaving your mind open" to the possibility that such compromising information exists.
Trump and Comey traded barbs before, but Trump's Friday tweets are the first since details of Comey book have been revealed. "That's not a perspective, that's not a view, that's just a lie". The firing ultimately led to the appointment of Robert Mueller to direct a special counsel inquiry into Russian meddling in the 2016 US presidential election and potential collusion between the Trump campaign and the Kremlin.
Ayone who can see "moral equivalence" in protests by white supremacists and people who oppose them, who treats women like "pieces of meat" and who "lies constantly" and insists Americans believe it is "not fit to be president of the United States on moral grounds", Comey said in an interview on ABC.
"The audience could see my face change", he said. Trump spoke in monologues, he said, but did not listen.
Trump wrote early on Sunday in one of five Twitter posts aimed directly at Comey. Asked whether Comey was a man of integrity, the Republican speaker said: "As far as I know", but added that he did not know him well.
"The lying about all things, large and small, in service to some code of loyalty that put the organisation above morality and above the truth", Comey said. I'm not trying to, by the way, suggest that President Trump is out breaking legs and-you know, shaking down shopkeepers. I can not say that (Trump is not subject to Russian blackmail).
Hours before the interview aired, Trump launched into a preemptive tirade on Twitter berating Trump. His "memos" are self serving and FAKE!'