Trump immediately fired back on Twitter.
Speaking to students and guests at Drew University in New Jersey on Wednesday night, Kelly said that Vindman was just doing his job in reporting "an illegal order," migrants are “overwhelmingly good people” and “not all rapists,” and Trump’s decision to condition military aid to Ukraine on an investigation into his political rival disrupted longstanding U.S. policy.
Trump quickly responded by attacking the retired Marine Corps general, saying that he couldn’t have terminated Kelly fast enough and that he “came in with a bang, went out with a whimper.” He said Kelly “just can’t keep his mouth shut.”
But Trump did not directly address Kelly’s specific criticisms in which he attacked the president on several domestic and foreign policy issues throughout the 75-minute appearance.
Regarding Trump's controversial July 25 phone call with Ukraine's president, Kelly said it changed longstanding U.S. policy and that Vindman reported it because is was tantamount to hearing "an illegal order."
“Through the Obama administration up until that phone call, the policy of the U.S. was militarily to support Ukraine in their defensive fight against … the Russians,” Kelly said. “And so, when the president said that continued support would be based on 'X,' that essentially changed. And that’s what that guy [Vindman] was most interested in.”
“[Vindman] did exactly what we teach them to do from cradle to grave,” Kelly continued. “We teach them: Don’t follow an illegal order. And if you’re ever given one, you’ll raise it to whoever gives it to you that this is an illegal order, and then tell your boss.”
Kelly also criticized Trump's handling of North Korea, saying Trump's efforts to end the regime's nuclear program through diplomacy are futile.
“[Kim Jong Un] will never give his nuclear weapons up,” Kelly said. “Again, President Trump tried -- that’s one way to put it, but it didn’t work. I’m an optimist most of the time, but I’m also a realist, and I never did think Kim would do anything other than play us for a while, and he did that fairly effectively.”
Kelly also said he didn't believe the press is “the enemy of the people,” and sharply criticized Russian President Vladimir Putin as someone who sits atop “a society in collapse,” yet is intent on restoring “the glory days of the Soviet Union."
He denounced as well Trump's intervention in the case of Eddie Gallagher, a Navy Seal who was convicted last year of posing for photos with the corpse of an ISIS fighter.
"He was found guilty of certain thing," Kelly said of Gallagher. He "should have been ashamed of himself and he should have been sent home. So, the idea that the commander in chief intervened there in my opinion, was exactly the wrong thing to do. And had I been there, I think i could have prevented it," Kelly added.
"I was disappointed," White House press secretary Stephanie Grisham said on Fox News Thursday morning. "... I saw some of the comments that he made and I was in the room with him when he actually backed the president on many of the things that he is now saying is weren't great."
She pushed back specifically on his remarks defending the press, saying she heard Kelly make negative comments about the news media.
Bolton came to Kelly's defense Thursday afternoon on Twitter, calling his former colleague "an honorable man."
"John Kelly is an honorable man. John and I have disagreed at times, as is commonplace at senior government levels, but he has always served his country faithfully. Conservatives especially have a responsibility to reject baseless attacks upon him," Bolton tweeted.
Kelly had defended Bolton just two weeks ago at at a lecture series in Sarasota, Florida, telling an audience he believes the accounts detailed in Bolton's forthcoming book. According to reported excerpts from the book, Bolton claims that Trump conditioned military aid to Ukraine on Ukraine agreeing to investigations into his political rival Joe Biden. Trump has vehemently denied Bolton's reported claims.
“If John Bolton says that in the book, I believe John Bolton,” Kelly said in late January. “Every single time I was with him ... he always gave the president the unvarnished truth” he added.
This is not the first time the two men have exchanged harsh words.
Trump attacked Kelly last October, after he said he had warned the president that hiring a “yes man” to replace him would lead to impeachment.
“John Kelly never said that, he never said anything like that," Trump said in the statement, released by the White House on Oct. 26, 2019. "If he would have said that I would have thrown him out of the office. He just wants to come back into the action like everybody else does.”
Although their relationship has been strained since Kelly's departure, Trump told reporters around the time of Kelly's December 2018 resignation, “John Kelly will be leaving. I don’t know if can say ‘retiring’, but he’s a great guy.”
Following his time in the Trump White House, Kelly joined the board of Caliburn International, the parent company of Comprehensive Health Services, which operates shelters for unaccompanied migrant children.
ABC News' Katherine Faulders contributed to this report.