Khan's approval of the 19-foot blimp, paired with other protests activists have planned in London during Trump's visit, could add tension between the two leaders who have long sparred over their approaches to national security.
As anti-Trump activists demonstrate during the U.S. president's visit, supporters will gather in the British capital to welcome him to Britain.
Tens of thousands of people are expected to march through London in protest against Mr Trump's visit and some of his actions during his first 18 months in power.
Air Force One landed safely at Stansted Airport at 13.52 after flying in from Brussels where the U.S. president had attended the North Atlantic Treaty Organisation summit.
Trump - on his first visit to the country after taking office - will attend a black-tie dinner hosted by Prime Minister Theresa May and have tea with Queen Elizabeth at the Windsor Castle before flying off to one of his golf clubs in Scotland.
He said: 'They like me a lot in the UK.
Johnson added that the President "appreciates free speech" and that he was "very focused on the special relationship" between the USA and the UK.
And with a giant "Trump Baby" balloon set to fly in the air over multiple demonstrations, US officials urged Americans to "keep a low profile" in the English capital.
The relation between the two trans-Atlantic allies has seen a bumpy ride in recent times but despite that, the British government hoped to make a quick trade agreement with the U.S. after it goes out of the European Union.
"It's typical, but honestly Donald Trump's legacy will outlive any big orange balloon and the matters that will be discussed at Chequers and now in Brussels are much more important and will have a much longer effect than any day rally", she said.
It appears the US President isn't looking forward to his visit to the United Kingdom very much - branding the country as in "turmoil".
Trump is technically on a lower-level working visit this week.
May was the first head of government to officially meet Trump as president, in January 2017.
Trump has clashed in the past with May - even though she is a conservative who shares his view that defense spending should be raised - and with her predecessor, David Cameron, who challenged Trump's anti-Muslim campaign stance as "divisive, stupid and wrong".
Thousands are expected to protest across the capital, heavily led by Owen Jones" "Stop Trump' rally, which will go ahead on Friday July 13. It was during that trip to Washington that she invited him to the UK.
The trip coincides with a tumultuous week for May after two senior ministers resigned in protest at her plans for trade with the European Union after Britain leaves next March.