His revelation comes as officials defended the government over procuring equipment essential in saving lives from coronavirus.
The prime minister has been urging businesses to ramp up production of the life-saving machines which are currently desperately lacking in the UK.
But Trump invoked Korean War-era legislation to order the General Motors car giant to switch production lines to make ventilators.
The president said the UK PM issued an instant plea to him regarding the machines when they spoke after Johnson’s positive test for Covid-19.
“If we make too many that’s going to be OK because I spoke to the prime minister of the UK yesterday.” said Trump.
“And before I even was able to get a word out of him... I said, ‘How you doing?’ and he said, ‘We need ventilators’.
“The UK needs ventilators, a lot of countries need ventilators, badly.”
Donald Trump also revealed he is considering imposing a quarantine on New York to slow down the spread of coronavirus cases.
Downing Street said 8,000 additional ventilators had been ordered by the government to boost the stock of 8,000 already available to the health service.
But with Covid-19′s peak expected to strike the UK in around three weeks, officials said thousands may not arrive for a number of months.
Meanwhile, the cabinet office issued a detailed rebuttal of claims the government had missed opportunities to procure ventilators.
Andrew Rayner, the managing director of MEC Medical, which makes ventilator parts, told the Financial Times that he could have delivered 500 ventilators “if they got back to me straight away”, adding: “They’ve missed the boat.”
But the government department took the unusual step of rebutting “a number of inaccurate claims”.
“We would expect suppliers of ventilator parts, like MEC Medical, to receive orders from companies for some of the components needed to scale up production of UK ventilator manufacturers,” a statement said.
This article originally appeared on HuffPost.