In Trump's first face to face meeting with Johnson since his election as prime minister in July, he praised the pro-Brexit leader as "the right man for the job" of finally engineering the U.K.'s separation from the European Union.
Later on, he held a session with Donald Tusk, the outgoing EU President, who looked very glum after the Prime Minister told him that most of the 39 billion divorce bill won't be paid if there is no deal.
Facing a delicate task of assuaging European allies while not angering Trump at a G7 summit in France, Johnson said there were huge opportunities for British businesses in the USA market, but hinted at differences between the two sides on the scope of a deal.
The U.S. and United Kingdom are expected to finalize a trade deal sometime after Britain exits the EU.
Britain was looking forward to some comprehensive talks about taking the future UK-U.S. relationship forward, Johnson said, adding he had made clear to Trump that the National Health Service would not be a part of trade talks.
"I think it's going to be touch and go", he told the BBC.
"Actually we're getting along very well with China right now", Trump said.
"There remain very considerable barriers in the U.S. to British businesses which are not widely understood", he said, adding that he had raised this in telephone discussions with Trump, and would do so again when they meet for bilateral talks on Sunday morning.
Asked what his advice was for Brexit, Trump replied: "He needs no advice".
China warns U.S. to stop 'wrong' trade actions or face consequences
If Trump goes ahead with all the tariffs he's announced, they would cover just about everything China ships to the United States . Trump escalated "trade frictions" and is "seriously threatening the multilateral trading system", the Finance Ministry said.
G-7 summits, gathering Britain, Canada, France, Germany, Italy, Japan and the United States, were once a meeting of like-minded allies.
Johnson claimed food shortages - one of the risks outlined in the leaked Operation Yellowhammer documents on no-deal planning - were "highly unlikely", and offered a "guarantee" that patients would be able to access medicines unhindered.
He was speaking shortly before holding talks with Prime Minister Shinzo Abe on the side-lines of the G7 summit.
He said a "very big trade deal, bigger than we've ever had", will happen "quickly". -China trade war he was "very worried" about the growth of protectionism.
Meanwhile, the PM is to warn Brussels that he will cut more than £30 billion from the European Union divorce bill in the event of a no-deal Brexit, The Sunday Times reports.
"Just to register a faint, sheep-like note of our view on the trade war - we are in favor of trade peace on the whole", Johnson said.
"It is important that it continues and I don't think you will find any change in the British government position".
The British leader will also have a midday meeting with EU Council President Donald Tusk after a bitter verbal spat the day earlier.