The president's Twitter attacks followed leaked memos that Mr Darroch sent to the USA about Mr Trump, in which the United Kingdom ambassador described his administration as "uniquely dysfunctional" and "inept".
In an unprecedented public spat in the transatlantic alliance, Mr Darroch was branded "wacky" and a "stupid guy" by Mr Trump, who said the USA government would no longer deal with the UK's man in Washington after cables were published describing his White House as "inept" and "dysfunctional".
Darroch had been due to step down at the end of the year. "The current situation is making it impossible for me to carry out my role as I would like", Sir Kim said in his resignation letter. Mr. Trump not only lashed out at Sir Kim, he also said that the White House would no longer deal with the diplomat, effectively cutting off his Washington contacts.
When Trump stunningly defeated Clinton, ambassadors and diplomats from other foreign embassies came to Darroch asking for his help getting to know the incoming administration.
Mr Johnson made the comments in December 2015, after Mr Trump had claimed while on the Presidential campaign trail that London had "no go" zones that even the police would not enter.
Reports suggest that it was at that point that Darroch decided that he must resign.
Mr Johnson, who reportedly spoke to the ambassador by telephone yesterday afternoon, also expressed his regret, adding: "I hope that whoever it is, is run down, caught and eviscerated, quite frankly, because it is not right that advice to ministers that civil servants must be able to make in a spirit of freedom should be leaked".
Scotland's First Minister Nicola Sturgeon, the leader of the separatist Scottish National Party, said Darroch had been "forced out" for doing his job.
And while Boris Johnson's hesitation in backing him was a factor in Darroch's resignation, it was not the main reason. In the words of the chairman of the foreign affairs select committee, Tom Tugendhat, a former army officer: "Leaders stand up for their men". It called the leak "mischievous behavior" and said the public has expected diplomats to provide honest assessments of the politics in the countries where they're posted. After the leak over the weekend, UK Prime Minister Theresa May called the situation regrettable.
Britain prides itself on its "special relationship" with its closest ally and Trump himself described it as "the greatest alliance the world has ever known" during a lavish state visit to Britain last month.
There has been speculation that the leak was aimed at forcing out Darroch and replacing him with a more Brexit-friendly candidate.
Mr Johnson was however defended by supporter ex-Defence Secretary Michael Fallon, who attacked Mr Hunt backers for "a shabby attempt to politicise" the resignation.
"Time (to) put in a non-Remainer who wants a trade deal with America", he tweeted.