Donald and Melania were welcomed by Queen Elizabeth, the Prince of Wales and the Duchess of Cornwall in Buckingham Garden before dashing off for lunch at the palace.
The pair will later be joined by the Duke of York for a visit to Westminster Abbey ahead of a State Banquet in the evening, where they will meet Prince William and Kate Middleton.
The 72-year-old caused a stir ahead of his arrival with a tweet criticising at London Mayor, Sadiq Khan.
"@SadiqKhan, who by all accounts has done a terrible job as Mayor of London, has been foolishly 'nasty' to the visiting President of the United States, by far the most important ally of the United Kingdom. He is a stone cold loser who should focus on crime in London, not me. Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a terrible job - only half his height. In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now!"
Trump also denied calling Meghan Markle "nasty" ahead of his highly controversial visit.
"I never called Meghan Markle 'nasty'," he tweeted, adding: "Made up by the Fake News Media, and they got caught cold!"
Mr Trump made his remarks about the Duchess of Sussex after being quizzed about comments made by Meghan Markle ahead of the 2016 election.
Speaking on "The Nightly Show with Larry Wilmore" in 2016, Meghan called Trump misogynistic and said his politics are divisive, saying she would move to Canada if he won the presidency.
Trump told the Sun, "I didn't know that she was nasty. I hope she is OK. ..." He seemed to bury the hatchet saying, "I am sure she will go excellently (as a royal). She will be very good."
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