Ms Haley defended staffers in the Trump administration as loyal and respecting.
The ambassador's defence of Mr Trump comes after the publication of the book titled "Fire and Fury: Inside the Trump White House" by journalist Michael Wolff, in which he claims to have interviewed more than 200 people familiar with the purported chaos inside the Oval Office.
In the book, Mr Wolff writes that people around Mr Trump regularly question his intelligence and fitness for office.
"I know those people in the White House. These people love their country and respect our president... No one questions the (mental) stability of the president," Ms Haley said yesterday.
"I can't vouch for anything like that. I don't know if it was 200 interviews with Steve Bannon, or if it was 200 interviews with himself, but I can tell you, I know these people. I work with these people," she said.
Mr Trump, 71, has repeatedly slammed the book as fake, describing it as "full of lies."
"Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book. He used Sloppy Steve Bannon, who cried when he got fired and begged for his job. Now Sloppy Steve has been dumped like a dog by almost everyone. Too bad!" Mr Trump said in a tweet.
"I work with the president and speak with him multiple times a week. He didn't become the president by accident," Mr Haley said.
"We need to be realistic at the fact that every person, regardless of race, religion, or party, who loves the country, should support this president. It's that important," she said.
Ms Haley said she was in constant communication with not only with the president but also with people around him.
"I'm around them all the time. I see these people put everything they have got into their jobs and into respecting and trusting the president. If they didn't, they wouldn't be there," she said.
"Throughout my life, my two greatest assets have been mental stability and being, like, really smart," Mr Trump tweeted on Saturday.
Earlier, White House aide Stephen Miller described the book "a grotesque work of fiction".