"I didn't know how to conduct my affairs with him any other way than in a very straightforward fashion, and I think he grew exhausted of me being the guy every day that told him, 'You can't do that, '" he said. "He was lazy as hell".
Tillerson, 66, was sacked from the White House in March after rumors of a rift between him and Trump had been circulating for some time.
Trump fired back on Twitter, saying Tillerson "didn't have the mental capacity needed" as the nation's top diplomat. "It violates the law". "It's really a concern I have about us as Americans, and us as a society, and us as citizens".
"He acts on his instincts; in some respects, that looks like impulsiveness", Tillerson said.
Trump fired Tillerson in March after a series of public rifts over North Korea, Russia and Iran policy, dismissing the former Exxon Mobil Corp chief executive in a tweet.
Asked how his relationship with Trump had gone off the rails, Tillerson said it might have been the result of his telling the president that he could not do certain things because they were illegal or would violate USA treaties.
That led to very public disagreements that undermined Tillerson's ability to speak for the administation overseas and angered the president: "He grew exhausted of me being the guy every day that told him, 'You can't do that, and let's talk about what we can do, '" Tillerson said. Next to his other comments, though, it wasn't hard to see he was lumping Trump with all those Americans who aren't intellectually curious enough about policy and the actual details of USA government.
"I will be honest with you, it troubles me that the American people seem to want to know so little about issues, that they are satisfied with 128 characters" to get their news or receive statements from the president, he added.
The president tweeted praise for his current secretary of state, Mike Pompeo, while directing a torrent of abuse toward his predecessor.
Reports later surfaced that, actually, Tillerson had called Trump, "a f**king moron".