He said Mr Johnson's aim of getting a trade deal with no tariffs or quotas was "insufficient" and that the first "moment of truth" to assess whether more time was needed for trade talks would come next summer. He'd be so bad, he'd take you in such a bad way.
But the president, whose impeachment in the United States moved a stage closer on Thursday following a key vote in Congress, also criticised Johnson's Brexit deal, saying it made it impossible to strike a future trade agreement.
"To be honest with you, this deal, under certain aspects of the deal, you can't do it".
Speaking to Farage on radio station LBC, Trump slammed Corbyn and praised Johnson as "a fantastic man" - but urged Britain's Conservative leader to make an electoral pact with Farage's Brexit Party.
Trump told Farage he should be concerned about the possibility of Labor Party leader Jeremy Corbyn prevailing in the vote.
All this on the very day Britain was supposed to be coming out of the EU. "And I think he's willing to do what no one else would do", Mr Trump said.
But he was forced to ask the European Union to delay Brexit until January 31 after MPs in the House of Commons refused to approve the withdrawal terms he had struck with Brussels. Earlier this week, the European Union granted Britain a three-month Brexit delay, setting a new January 31 deadline for the country to leave and imploring British politicians to use the extra time wisely.
Pro-EU campaigners and business executives have breathed a sigh of relief that Britain avoided a Halloween Brexit nightmare that could have seen it crash out of the EU after 46 years without a plan.
Johnson had vowed he would rather be "dead in a ditch" than tolerate another extension to the tortuous process, which began in 2016 after a knife-edge public referendum.
For the first time in 18 years I won't be a candidate in the next General Election. "The time we have at hand to conclude this negotiation will be extremely short, 11 months".
"I know that you and him will end up doing something that could be terrific if you and he get together as, you know, an unstoppable force", Trump added.
The Prime Minister knew he didn't have the numbers in parliament to allow him to keep these promises, and to what extent the British electorate will now feel Mr Johnson was willfully misleading them with these repeated dishonest claims will be revealed at the ballot box.
But a National Institute of Economic and Social Research study suggested Wednesday that Johnson's Brexit deal could leave Britain pound sterling70 billion worse off in 10 years.
Divisions over Brexit have seen sometimes toxic rhetoric on all sides.
All seats in the 650-seat House of Commons are up for grabs in the early election, chosen by Britain's 46 million eligible voters. Death threats against lawmakers and attacks on social media have risen in recent years.