"If you've never been in combat, you can't even imagine how to make that call," Kelly told reporters in a surprise appearance at the daily White House press briefing.
Kelly, a Gold Star father, delivered an emotional defense of the president, who was criticized on Wednesday for his comments on a call with the widow of Sgt. La David Johnson, one of four Americans killed in Niger on Oct. 4.
"There's no perfect way to make that phone call," Kelly said, adding that when Trump asked him how to call the families of the four slain service members, "My first recommendation to him was that he not do it, because it's not the phone call that parents and family members are looking forward to."
Kelly described how Trump had asked him, "What do I say?" Kelly said he replied that there was nothing the president could say that would lighten the burden for the four families.
But to help Trump prepare for the calls, Kelly explained to the president what he himself had been told in a phone call after his son Robert Kelly was killed in combat in Afghanistan.
"He was doing exactly what he wanted to do when he was killed," Kelly said, recalling what his best friend, Gen. Joe Dunford, now chairman of the Joint Chiefs of Staff had told him. "He knew what he was getting into by joining that one percent. He knew what the possibilities were, because we were at war. And when he died he was surrounded by the best men on this earth."
Kelly added, "That's what the president tried to say to the four families."
Dunford's line about a soldier knowing what he was getting into is precisely what Trump tried to say to Myeshia Johnson in a phone call on Tuesday, first described publicly by Rep. Frederica Wilson, D-Fla., who had known the Johnson family for years and was sitting with Johnson when she received the call.
Wilson said she heard Trump tell Johnson that her late husband, "knew what he was signing up for .... but when it happens it hurts anyway," and that the president's call left Johnson in tears. Wilson was in a car with Johnson when the call came and only heard it after Johnson put it on speakerphone.
Nonetheless, Kelly sharply criticized Wilson for listening to the call. "It stuns me that a member of Congress would've listened in on that conversation," Kelly told reporters. "I thought at least that was sacred."
The duty of presidential condolence calls has been complicated in recent days by Trump's decision to compare the calls he's made with those of his predecessor, President Barack Obama. Earlier this week, Trump claimed that Obama had not called families of the fallen and that he had called the families of every soldier killed during his presidency.
Neither statement was true, and both Trump and White House press secretary Sarah Huckabee Sanders have walked back Trump's claims since then.
Kelly also appeared to refute Trump's insistence that Wilson had "totally fabricated" her account of the call, and that Trump had "proof" of that.
Asked by reporters Wednesday what he had talked to Johnson about, Trump quickly grew defensive.
"I didn't say what that congresswoman said; didn't say it all," Trump said at the White House. "I did not say what she said, and I'd like her to make the statement again because I did not say what she said. Had a very nice conversation with the woman, with the wife, who was — sounded like a lovely woman. Did not say what the congresswoman said, and most people aren't too surprised to hear that," Trump said.