This was their first one-on-one meeting after Johnson became Prime Minister late last month.
Trump told the reporters that a deal with the UK would happen "quickly".
"We're going to do a very big trade deal, bigger than we've ever had with the UK.
"And now at some point they won't have the obstacle, they won't have the anchor around their ankle, because that's what they have," he added.
Before his talks with the US President, Johnson however, said that the US must open up its markets if a post-Brexit trade deal was to be agreed.
"I don't think we sell a single joint of British lamb in the US, we don't sell any beef," the Prime Minister said.
Johnson spoke about "massive opportunities for the UK to prise open the American market".
As a member of the EU, the UK cannot make its own trade deals with other countries - and the EU does not have a free trade deal with the US.
The UK has already agreed 13 "continuity" deals with 38 countries that will apply post-Brexit.
Johnson's breakfast meeting with Trump came before a day of discussions with other world leaders at the summit, reports the BBC.
The Prime Minister will meet European Council President Donald Tusk, a day after the two men gave clashing statements over who would be held responsible for a no-deal Brexit.
With Brexit on the agenda when Johnson meets Tusk, Downing Street dismissed as "entirely false" newspaper suggestions the government was considering suspending Parliament "in order to stop MPs debating Brexit".