Months of unremitting political turbulence spiked on Tuesday when his health secretary and chancellor quit within 10 minutes of each other over the PM's handling of sexual misconduct allegations against a senior MP.
This triggered a trickle of resignations and letters of no confidence from junior ministers and MPs that on Wednesday became a flood. That evening, a group of senior ministers went to Downing Street to try to persuade the PM to resign.
Mr Johnson initially remained defiant and on Wednesday evening said he had no intention of resigning given his "colossal mandate" from voters at the 2019 election. Cabinet ally Jacob Rees-Mogg dismissed the campaign against him as a "squall".
But by Thursday morning more than 50 members of the government had quit, including Education Secretary Michelle Donelan who had only been appointed on Tuesday night. and Mr Johnson was forced to bow to the inevitable.
In a short statement, the opposition leader Sir Keir Starmer wrote that Mr Johnson's resignation "is good news for the country" and said he was "responsible for lies, scandal and fraud on an industrial scale".
So how did we get here?
A drunken night out in Westminster Last Thursday Noa Hoffman, a 24-year-old just four days into her job as a political reporter for the Sun newspaper, broke the news that a Conservative MP had resigned from his role as a party whip following a boozy evening at the Carlton private members club.
In his resignation letter, Chris Pincher told the PM he "drank far too much" and "embarrassed me and other people".
But the matter turned out to be far more serious: he is alleged to have grabbed two men at a private members' club, touching at least one of them on the groin.
Two things then happened. The government briefed the media that Mr. Pincher had recognized he had behaved badly and so would keep his job as an MP and face no further action. But behind the scenes Conservative MPs were furious.
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Similar allegations of sexual misconduct had been made against Mr. Pincher in the past. Yet Mr. Johnson had put him into a position of power as the party's deputy chief whip - enforcers who ensure MP discipline but who are also responsible for pastoral matters. Source: BBC
